Manual discharge gauging was carried out with either a Columbia (± 4%) or. General Oceanics Flowmeter ...... Bathymetry was mapped using a Garmin GPS and. Humminbird depth sounder ...... Limnology and Oceanography 20: 657-660.
SEDIMENTARY PROCESSES AND ENVIRONMENTAL SIGNALS FROM PAIRED HIGH ARCTIC LAKES
by Jaclyn Mary Helen Cockburn
A thesis submitted to the Department of Geography In conformity with the requirements for the degree of PhD
Queen’s University Kingston, Ontario, Canada (September, 2008)
© Copyright, Jaclyn Cockburn, 2008
Abstract Suspended sediment delivery dynamics in two watersheds at Cape Bounty, Melville Island, Nunavut, Canada were studied to characterize the hydroclimate conditions in which laminated sediments formed. Process work over three years determined snow-water equivalence was the primary factor that controlled sediment yield in both catchments. Cool springs (2003, 2004) enhanced runoff potential and intensity because channelized meltwater was delayed as it tunneled through the snowpack and reached the river channel (sediment supply) within 1-2 days.
In warm springs (2005), meltwater channelized on the
snowpack and did not immediately reach the river bed (7-10 days). Sediment transport was reduced because flow competence was lower and sediment supplies limited. Sediment deposition in the West Lake depended on surface runoff intensity.
Short-lived, intense episodes of turbid inflow generated underflow
activity which delivered the majority of seasonal sediment. In 2005, runoff was less intense and few underflows were detected compared to the cooler, underflow dominated 2004 runoff season. As well, grain-size analysis of trapped sediment indicated that deposition rates and maximum grain-size were decoupled, indicative of varied sediment supplies and delivery within the fluvial system.
These decoupled conditions have important implications for
paleohydrological interpretations from downstream sedimentary records.
ii
Two similar 600-year varve records were constructed from the lakes at Cape Bounty. Although these series were highly correlated throughout, timedependent correlation analysis identified divergence in the early 19th century. Because the varve records were from adjacent watersheds and subject to the same hydroclimatic conditions, the divergence suggests watershed-level changes, such as increased local active layer detachments. The varve record from West Lake was highly correlated with lagged autumn snowfall and spring temperature. Similar relationships between these variables and East Lake were not as strong or significant. Long-term climatic interpretations should be carefully assessed. A single record from either of these lakes might lead to autumn snowfall and/or springmelt intensity reconstructions, given the process work and weather record correlations. The recent divergence reveals potential changes likely to occur as warming increases variability within the Arctic System.
Multidisciplinary
monitoring and observations should continue in order to quantify future variability and evaluate the impact on these systems.
iii
Co-Authorship Lake trap collection and instrument deployment was planned and coordinated by the author with assistance by Scott Lamoureux and all Cape Bounty field camp members in 2003, 2004 and 2005. River sample collection was planned and coordinated by the author, with major assistance by Scott Lamoureux, Andrew Forbes, Dana McDonald and Elizabeth Wells (2004), along with collection assistance from all Cape Bounty field camp members in 2003, 2004 and 2005. Snow water equivalence measurements were collected by Andrew Forbes (2003), Krys Chutko (2004), Melissa Lafrenière and Brock Macleod (2005). Analysis of the snow data was carried out by Melissa Lafrenière, Brock Macleod, Elizabeth Wells and Scott Lamoureux. Meteorological data were collected by Scott Lamoureux with assistance from the Cape Bounty field teams.
Long
sediment cores and bathymetric data were collected in 2003 with major assistance from Scott Lamoureux and Andrew Forbes. Several more sediment cores and bathymetric data sets were collected in 2004 with the assistance of Krys Chutko, Dana McDonald and Elizabeth Wells and in 2005 with the assistance of Scott Lamoureux and Jessica Tomkins. All laboratory and data analyses for Chapters 2 - 4 were carried out by the author. Members of the EVEX laboratory in the Geography Department and PEARL group in the Biology Department at Queen’s University assisted in the timely completion of the analyses.
iv
Acknowledgements I have many people to thank for all the encouragement and support I have received through my time at Queen’s and in the Geography Department.
I
cannot possibly do everyone justice here, but know that you have made a difference, and without your support this would not have been possible. I would like to thank Scott Lamoureux for his supervision and encouragement through my PhD. His knowledge and expertise seem endless and his enthusiasm for all things cold and muddy is contagious. I am a better scientist and a better teacher for having known him. Through my umpteen years as a student at Queen’s I also became close to his family and would like to thank Linda, Mackenzie and Brenna for always welcoming me and making me smile. To Bob Gilbert – thanks for taking a chance back in 1999 and hiring me as a summer student. I look back on that summer with fondness and know that I wouldn’t be where I am today without that opportunity.
Your passion and
imagination for the physical environment are inspiring. To the Polar Continental Shelf Project in Resolute – the high Arctic is an amazing place, with your support, expertise and good humour, you made this work possible and fun. Thanks to all the staff through the years that have helped and continue to help the work at Cape Bounty. To Jess, Krys and David – I can’t thank you guys enough.
There is
something to say for safety in numbers. Whether it was a coke, more coffee or a chat over backgammon you helped make this a great experience for me. v
To everyone who has been to Cape Bounty, shipped stuff to Cape Bounty or had to find it on the map, thanks. I would especially like to thank the members of the Cape Bounty field campaigns in 2003, 2004 and 2005. To members of the EVEX, LARSEES and PEARL research groups, thank you for your assistance in field and sample prep. To my family and friends – words are not enough to describe my gratitude. Thank you for being there and supporting me always.
vi
Statement of Originality I hereby certify that all of the work described within this thesis is the original work of the author. Any published (or unpublished) ideas and/or techniques from the work of others are fully acknowledged in accordance with the standard referencing practices.
Jaclyn Cockburn
(September, 2008)
vii
Table of Contents Abstract ............................................................................................................................. ii Co-Authorship .................................................................................................................. iv Acknowledgements ........................................................................................................... v Statement of Originality................................................................................................... vii Table of Contents............................................................................................................viii List of Figures.................................................................................................................... x List of Tables.................................................................................................................... xi Chapter 1 Introduction.......................................................................................................1 Chapter 2 Hydroclimate controls over seasonal sediment yield in two adjacent High Arctic watersheds..............................................................................................................5 2.1 Abstract ...................................................................................................................5 2.2 Introduction..............................................................................................................6 2.3 Study Site ................................................................................................................9 2.4 Methods.................................................................................................................11 2.4.1 Meteorology ....................................................................................................12 2.4.2 Hydrology........................................................................................................13 2.5 Results...................................................................................................................17 2.5.1 Hydrometeorology...........................................................................................17 2.5.2 Sediment Delivery ...........................................................................................24 2.6 Discussion .............................................................................................................25 2.6.1 Hydroclimate controls over seasonal runoff ....................................................25 2.6.2 Hydroclimate controls on seasonal sediment delivery ....................................29 2.6.3 Sensitivity of sediment yield to climate variability in high arctic watersheds ...35 2.6.4 Interpreting hydroclimatic variability from downstream sedimentary records..37 2.7 Conclusions ...........................................................................................................41 Chapter 3 Inflow and lake controls on short-term mass accumulation and particle size in a High Arctic lake: implications for interpreting varved lacustrine sedimentary records..44 3.1 Abstract: ................................................................................................................44 3.2 Introduction............................................................................................................45 3.3 Study Site ..............................................................................................................46 3.4 Methods.................................................................................................................49 3.4.1 Hydrometeorology...........................................................................................49
viii
3.4.2 Limnology........................................................................................................51 3.5 Results...................................................................................................................54 3.5.1 Hydrometeorology...........................................................................................54 3.5.2 Sediment deposition rates and patterns .........................................................63 3.5.3 Sedimentary grain size characteristics............................................................70 3.6 Discussion .............................................................................................................74 3.6.1 Short-lived deposition patterns in mass accumulation and vertical distribution .................................................................................................................................74 3.6.2 Implications for sedimentary grain size interpretations ...................................82 3.6.3 Interpreting the sedimentary record from West Lake and similar settings ......84 3.7 Conclusions ...........................................................................................................86 Chapter 4 Snowfall variability and post-19th century arctic landscape disturbance revealed by paired varved sedimentary records .............................................................87 4.1 Abstract .................................................................................................................87 4.2 Introduction............................................................................................................88 4.3 Study Site and Methods ........................................................................................90 4.4 Results...................................................................................................................93 4.5 Discussion .............................................................................................................99 4.5.1 Divergent varve records ..................................................................................99 4.5.2 Hydroclimatic record .....................................................................................102 4.6 Conclusion...........................................................................................................104 Chapter 5 Conclusions and Future Work ......................................................................106 5.1 Summary .............................................................................................................106 5.2 Future Work.........................................................................................................109 5.3 Conclusion...........................................................................................................110 References....................................................................................................................112 Appendix A Correlation between Mould Bay and Rea Point weather stations..............129 Appendix B Suspended sediment trapping in limnological process studies .................130
ix
List of Figures Figure 2.1: Location map of Cape Bounty on the southern coast of Melville Island in the Canadian High Arctic. ..............................................................................................10 Figure 2.2: The effect of sample density on estimating total seasonal suspended sediment yield in the East River, 2005.....................................................................16 Figure 2.3: Daily mean temperature at Rea Point, Mould Bay and Cape Bounty ...........19 Figure 2.4: Cumulative melting degree days at Cape Bounty ........................................20 Figure 2.5: Hourly hydrometeorological summaries...................................................21-23 Figure 2.6: Cumulative discharge and suspended sediment yield compared to cumulative melting degree days ..............................................................................27 Figure 2.7: Mean monthly June,and July air-temperature records from Mould Bay and Rea Point weather stations ......................................................................................40 Figure 3.1: Cape Bounty, Melville Island, Nunavut, and locations of meteorological and hydrological stations ................................................................................................47 Figure 3.2: Schematic of the suspended sediment trap system .....................................52 Figure 3.3: West Lake seasonal inflow and depositional summaries for 2003...............56 Figure 3.4: West Lake seasonal inflow and depositional summaries for 2004...............58 Figure 3.5: West Lake seasonal inflow and depositional summaries for 2005...............60 Figure 3.6: Ratios of lower trap sedimentation rates to upper trap sedimentation ..........65 Figure 3.7: The ratio of Proximal to Mid site sedimentation rates ...................................67 Figure 3.8: West Lake inflow and deposition between June 28 and July 10, 2004........69 Figure 3.9: Mean grain size and deposition rates in the lower traps ..............................72 Figure 3.10: Deposition rates versus mean grain size in traps .......................................73 Figure 3.11: Schematic representation sediment delivery and deposition .....................81 Figure 4.1: Coring sites in West and East Lakes at Cape Bounty...................................91 Figure 4.2: West and East varve thickness records ........................................................95 Figure 4.3: Time-dependent Pearson correlation coefficients........................................96 Figure 4.4: West and East varve thickness records for the 20th century ........................97
x
List of Tables Table 2.1: Differences in SSQ estimates based on spline curves...................................16 Table 2.2: Estimated snow water equivalence and total runoff for each watershed .......17 Table 2.3: Regression coefficients for daily discharge, suspended sediment yield and melting degree days for each river...........................................................................33 Table 3.1 Mean June temperature at Cape Bounty, snow-water equivalence (SWE), total discharge and suspended sediment yield................................................................55 Table 3.2 Total suspended sediment deposition in the upper and lower traps in the Proximal and Mid stations in West Lake ..................................................................64 Table 3.3 Specific suspended sediment delivery and deposition (Mid lower trap) in West River and Lake 2003-2005.......................................................................................70 Table 4.1: Pearson correlation coefficients between the varve thickness measurements and weather variables..............................................................................................93 Table 4.2: Pearson correlation coefficient between the varve records............................96 Table 4.3: F-test statistic for selected time periods .........................................................97
xi
Chapter 1 Introduction It is widely understood that the Earth’s climate varies naturally due to large-scale earth system processes. There is a consensus that human activities are altering atmospheric composition, which in turn will alter the earth’s climate system (IPCC 2007). The impact of anthropogenic climate change on earth system processes is wide in scope and in some cases not yet clearly understood, particularly in the Canadian High Arctic. The Canadian High Arctic has limited instrumental climate data available, which is problematic when considering long-term environmental variability in this region (ACIA, 2005). Understanding current changes in a broader context requires longer records of change. Proxy indicators or natural archives record past climate and environmental variations (Bradley, 1999) and when combined with modern climatological measures, provide the means to quantitatively calibrate and assess proxies with respect to present-day conditions. One common proxy, annually laminated lake sediments, referred to as varves, has the potential to reconstruct annual variations in hydroclimatic variability (e.g., Hardy et al., 1996; Overpeck et al., 1997; Hughen et al., 2000; Hodder et al., 2007).
Varve formation and
preservation occurs in a number of environmental circumstances, such as in lakes where seasonal sediment delivery and deposition are driven by river inflow and sediment transport (Sturm and Matter, 1978; Sturm, 1979; Smith, 1981). In most cases, varve thickness reflects, in part, variation in hydroclimatic behaviour 1
that determines runoff and transport of available material (Gilbert, 1975; Desloges, 1994; Desloges and Gilbert, 1994a,b; Lamoureux, 2002; Cockburn and Lamoureux, 2007; Hodder et al., 2007). Broadly, individual climate reconstructions based on one proxy have been combined to produce indices to compare with climate forcing mechanisms (e.g., Overpeck et al., 1997; Mann et al., 1998; 1999). Each of these multi-proxy paleoclimate reconstructions draws credibility from statistically significant signals extracted from the compiled records and correlated with recent measures of climate
forcing
mechanisms
(e.g.,
solar
irradiance,
atmospheric
CO2
concentrations: Overpeck et al., 1997; Mann et al., 1998; 1999). These multiproxy compilations demonstrate that there is a measurable common factor influencing individual records, and given the geographical extent over which these records correlate, it is assumed that the principle factor is related to climate. Spatial variability in processes is often used to explain poor correlations between different records. However, few studies attempt to demonstrate the impact that spatial variability may have on records because most focus on single records and thus preclude such analyses. In general, it is anticipated that there is an underlying signal or pattern that is reproducible at a high resolution (annual) from similar proxy records (e.g., varved lake sediments) from the same region. However, there are few studies that have compared annual proxy records (e.g., varves: Desloges, 1994; Hughen et al., 2000; Menounos et al., 2005; varves and tree-rings: Luckman, 2000) from 2
similar regions. In most cases, discrepancies between sedimentary records are attributed to location-specific factors (e.g., physiography, weather). However, there are rarely localized sedimentary process measurements that can substantiate the character and magnitude of these discrepancies, and thus, the impact of local differences on the individual records is unknown. Proxy records with annual resolution afford the best opportunity to compare the climate signal reproducibility from similar regions. The wellconstrained temporal resolution allows common forcing mechanisms (e.g., climate) to be identified. Furthermore, it allows available meteorological and hydrological records to be used for calibration and comparison processes. As well, seasonal process studies can be integrated into the calibration analyses to better understand the record (Hardy et al., 1996; Lewis et al., 2002). This study assesses annual reproducibility in two varve records from the Canadian High Arctic in order to understand what environmental signal is preserved.
Through a combination of field process measures and available
meteorological records, the mechanisms by which varve sediments are deposited in two lakes were assessed. Beyond the available instrument data, the two records were used to independently verify and validate the signal preserved in the varve record and identify anomalies due to geomorphic processes or other differences rather than regional hydroclimatic controls.
3
It is hypothesized that there is a strong underlying climate signal, reproducible at an annual scale between individual records from a similar region for the entire length of the record (i.e., regardless of post-industrial anthropogenic climate forcing mechanisms). In order to test the reproducibility of the dominant annual signal in individual records, paired reconstructions based on clastic varve deposition in two High Arctic lakes with adjacent watersheds were developed and compared. Although evidence indicates that this is difficult to achieve and the success of compilations tend to be at coarser temporal scales, previous studies have not closely calibrated seasonal sediment deposition with hydroclimatic measures or taken place in similar lake and watershed settings.
Through
multiple seasons of observations, this study evaluated the seasonal fluvial and lake sedimentary processes for each watershed. In doing so, the similarities between the adjacent systems were compared through the last six centuries.
4
Chapter 2 Hydroclimate controls over seasonal sediment yield in two adjacent High Arctic watersheds In Press, Hydrological Processes Authors: Jaclyn M.H. Cockburn Scott F. Lamoureux
Keywords: Nival melt; seasonal suspended sediment transfer; sediment delivery; snow water equivalence, climate, erosion
2.1 Abstract Interannual variations in seasonal sediment transfer in two High Arctic nonglacial watersheds were evaluated through three summers of field observations (2003-05). Total seasonal discharge, controlled by initial watershed snow water equivalence (SWE) was the most important factor in total seasonal suspended sediment transfer. Secondary factors included melt energy, snow distribution and sediment supply.
The largest pre-melt SWE of the three years studied
(2004) generated the largest seasonal runoff and disproportionately greater suspended sediment yield than the other years. In contrast, 2003 and 2005 had 5
similar SWE and total runoff, but reduced runoff intensity resulted in lower suspended sediment concentrations and lower total suspended sediment yield in 2005. Lower air temperatures at the beginning of the snowmelt period in 2003 prolonged the melt period and increased meltwater storage within the snowpack. Subsequently,
peak
discharge
and
instantaneous
suspended
sediment
concentrations were more intense than in the otherwise warmer 2005 season. The results for this study will aid in model development for sediment yield estimation from cold regions and will contribute to the interpretation of paleoenvironmental records obtained from sedimentary deposits in lakes.
2.2 Introduction Spring snowpack and thermal conditions determine the magnitude and intensity of runoff in Arctic rivers. Projected climate scenarios suggest that discharge in arctic rivers will increase due to greater precipitation (ACIA, 2005) and seasonal sediment discharge may also increase. These conclusions are consistent with modeling studies based on ungauged Arctic rivers of varying basin area and runoff magnitudes (Syvitski, 2002), but the sparseness of sediment delivery data from these regions is acute. In addition to predicted increases in discharge due to more precipitation, warmer temperatures may also increase sediment yield through increased freeze-thaw processes and frozen ground dynamics (Woo et al., 1992; Syvitski, 2002). Although models predict increased sediment yield, there are few multi-year studies from Arctic catchments available for comparison with model results. 6
In the Canadian High Arctic, non-glacial watersheds are characterized as nival streamflow regimes, with short-lived flow (approximately 70-100 days; Woo 2000) and maximum discharge generated by spring snowmelt (Church, 1972). Seasonal suspended sediment concentration (SSC) generally mirrors stream discharge patterns; thus, high concentrations typically occur during or just prior to the peak snowmelt runoff period (Woo and Sauriol, 1981; Lewkowicz and Wolfe, 1994; Forbes and Lamoureux, 2005). Furthermore, discharge magnitudes are limited by total snowpack and melt intensity, since the primary source for surface runoff is melting snow. Woo and Sauriol (1981) observed that cooler springs prolonged snowpack melt processes and generated greater snowpack meltwater storage within large snow banks and in channels filled with snow. The prolonged melt period delays and ponds meltwater, which, once released, can generate short-lived intense runoff that often accounts for a high proportion of the entire seasonal discharge (Woo and Sauriol, 1981; Hardy, 1996). This brief period of intense nival discharge generates high flow competence and fluid shear stress and thus the potential for higher suspended sediment erosion, transport and seasonal yield (Church, 1972; Lewkowicz and Wolfe, 1994; Forbes and Lamoureux, 2005). Thus total suspended sediment discharge (SSQ) or seasonal suspended sediment transported in a watershed is closely related to the intensity and duration of nival discharge (Q) for catchments with abundant sediment supply.
7
Multi-year studies (Lewkowicz and Wolfe, 1994; Priesnitz and Schunke, 2002; Forbes and Lamoureux, 2005) found that spring snow water equivalence (SWE) explained the overall magnitude of total runoff better than spring melt conditions (estimated by air temperature indices). This suggests that seasonal suspended sediment yield appears to be closely linked to spring SWE; consequently, snowpack exhaustion may limit total suspended sediment delivery in nival streams.
However, sediment supply variations that result in
intraseasonal sediment hysteresis can also play an important role in determining yield (Nistor and Church, 2005; Hasholt and Mernild, 2006), although relatively few studies of sediment yield hysteresis have been carried out in high latitude watersheds. For example, at Hot Weather Creek, Ellesmere Island, sediment supply appeared to be abundant and it was noted that sediment deposited in the channel-bed after the previous day’s peak waned was subsequently remobilized with increased discharge the following day (Lewkowicz and Wolfe, 1994). This study presents three seasons of sediment yield observations from two similar, adjacent watersheds in the Canadian High Arctic. This study aimed to distinguish primary hydroclimate controls over seasonal sediment delivery in similar watersheds. It was hypothesized that observed differences between the watersheds subject to similar hydroclimatic forcings would reveal the nature and magnitude of interseasonal suspended sediment yield hysteresis. In this manner the results of this study provide the first analysis of paired watershed climate-
8
sediment yield dynamics with implications for assessing future climate sensitivity and model verification.
2.3 Study Site Cape Bounty (74º55’N, 109º35’W, Figure 2.1) is located on the south-central coast of Melville Island, Nunavut, in the western Canadian High Arctic.
The
landscape is characterized by relatively simple drainage patterns, sparse tundra vegetation and continuous permafrost. The active layer varies between 20 and 70 cm depth and surface detachments and gullies are common features along the river channels.
The underlying bedrock of central Melville Island is
characterized by prominent syncline and anticline features (Harrison, 1994). The dominant bedrock type in the headlands consists of upper Devonian Beverley Inlet Formation and the middle Devonian Hecla Bay Formation is found in the lowlands. Both formations are characterized by heavily weathered sandstones and siltstones (Hodgson et al., 1984; Harrison, 1995).
9
Figure 2.1: Location map of Cape Bounty and the southern coast of Melville Island in the Canadian High Arctic. Inset map shows locations of Meteorological Service of Canada (MSC) stations at Rea Point, Mould Bay and Resolute (temperature only at Rea Point). Environmental monitoring stations and snow survey transect locations conducted each year are indicated. The transect network was expanded in 2004 and 2005. However, snow survey results presented in this study use the smaller 2003 subset for consistency.
10
Two adjacent watersheds with similar physiography were studied during the 2003-2005 melt seasons. The West and East River1 watersheds are 8.0 km2 and 11.6 km2, respectively (Figure 2.1). The uplands of both watersheds reach 110-125 m above sea level (a.s.l.) and are characterized as gently sloped plateaus covered in a veneer of glacial till and regressive Holocene marine sediments (Hodgson and Vincent, 1984; Hodgson et al., 1984). The West River has a slightly steeper gradient than the East River and as such, the West catchment has more frequent and well-expressed gullies compared to the East catchment. This region is classified as a polar desert characterized by cold winters, cool summers, and limited precipitation that occurs primarily as snowfall (Maxwell, 1981).
Mean summer (June, July, August) and winter (December,
January, February) temperatures at Rea Point (105 km northeast (Figure 2.1), 1969-1985) are 1.9 and –32.2ºC, respectively. Annual precipitation is dominated by snow in winter months (< 150 mm, Mould Bay, NWT); whereas summers are characterized by infrequent, low-intensity rainfall (< 10 mm/day).
2.4 Methods A comprehensive watershed research program was established in 2003 to monitor meteorological, hydrological and sediment transport conditions in both watersheds at Cape Bounty. Prior watershed observations from the region are
1
All river names are unofficial
11
limited to short intervals (e.g., Wedel et al., 1977; McLaren, 1981) or comprehensive data collection in small-scale slope studies (Lewkowicz and Young, 1990).
2.4.1 Meteorology
Three seasonal meteorological stations were established in June 2003.
The
primary station (MainMet) was located on the boundary between the two watersheds and an additional station was located in the headwaters of each watershed (Figure 2.1). Air temperature was measured 1.5 m above the ground with thermistors (accuracy 0.4°C) and recorded at 10-minute intervals with either Onset Hobopro (MainMet) or H8 loggers. Rainfall was measured with a Davis industrial tipping bucket gauge (0.2 mm resolution) and an Onset Hobo event logger at all three stations. Systematic wind, incoming solar and net radiation, and relative humidity measurements were also recorded at MainMet, but results are not described in this study. Snow surveys were completed in early June of each season and consisted of eleven depth measurements along 100-m transects with at least one density measurement per transect. Transects were established at 15 locations in 2003 and expanded to 23 and 41 locations in 2004 and 2005, respectively (Figure 2.1). Terrain classes were determined prior to the 2003 field season based on topographic maps and aerial photographs.
For purposes of
comparison between the three years, the results from the 2003 transect locations 12
are used in this study, although this necessarily reduces the available data. The terrain index method (Yang and Woo, 1999) was used to estimate watershed snow water equivalence (SWE) for each terrain class.
2.4.2 Hydrology
River gauging stations were established prior to runoff in each season at locations with minimal channel snow cover and a single well-defined channel (Figure 2.1).
Stage was measured with a Sensym SCX vented differential
pressure transducer recorded at 10-minute intervals with an Onset Hobo H8 logger (accurate to 2 mm) in 2003 and Omega CP-Level101 (± 0.2%, 0.5 mm) pressure transducer loggers with an Omega CP-PRTEMP101 (± 0.4% atmospheric pressure) logger for barometric compensation in 2004 and 2005. Manual discharge gauging was carried out with either a Columbia (± 4%) or General Oceanics Flowmeter (± 1%) to rate the streams throughout each season.
A minimum of 12 points were used to develop rating curves each
season (r2 = 0.796 – 0.905) that were combined with recorded stage measurements to construct seasonal hydrographs and calculate total season discharge (estimated ± 10%). Due to unfamiliarity with the stream channels and deep channel snowpack in 2003, the gauging station on the East River was initially located in the middle of the channel. The resulting stage record, which included the highest flow of the season, was deemed unusable because the stilling well caused flow to back up.
13
Suspended sediment concentration (SSC) was determined from filtered water samples collected with a DH-48 integrated water sampler at eight-hour intervals in 2003 (West 0100, 0900, 1700h; East 0000, 0800, 1600h local time), and hourly intervals during the peak snowmelt period, and two-hour intervals thereafter in 2004 and 2005 from the West River. In East River, 2004 and 2005 SSC samples were taken less frequently due to personnel limitations. Between 4 and 10 samples per day were collected in 2004 and between 3 and 6 samples per day in 2005. Volumetric samples were vacuum filtered with tared 0.45 µm cellulose acetate (2003) and 1.0 µm glass fiber filters (2004 and 2005) and reweighed twice after drying at 50ºC in the laboratory to determine suspended sediment concentration (± 0.1 mg·L). The filters were changed in 2004 to 1.0 µm glass fiber filters to increase field process capacity and sample collection. To evaluate the expected losses due to changing the type of filter after 2003, varying sediment concentrations were filtered with tared 1.0 µm glass fiber filters, the filtrate was then filtered with tared 0.45 µm cellulose acetate filters to estimate the loss associated with using the 1.0 µm glass fiber filters. In all cases, the difference between the 1.0 µm glass fiber and 0.45 µm cellulose acetate filters was minimal and does not represent a significant difference in the concentrations between years, but we are mindful that the SSC values obtained in 2003 may be slightly higher. The total suspended sediment discharge (SSQ) each season was calculated from point SSC samples and total discharge in each river. In order to 14
estimate SSC between point samples, spline curves constrained by the SSC point samples were used to construct an hourly sedigraph. From these values, SSQ was calculated at one hour intervals as the sum of the product of SSC and Q (± 20 kg·d-1).
Limited sample processing capacity in 2003 restricted SSC
samples to three per day for each river, while increased capacity in the subsequent years generated spline curves constrained by as many as 24 hourly point samples. In order to determine the bias induced by higher sampling resolutions in 2004 and 2005, alternative spline curves were fit with the minimum number of sample points (three samples daily as collected in 2003) from the 2004 and 2005 data in order to compare the estimated SSQ values for each season (Table 2.1). Due to the higher sampling frequency in 2004 and 2005, the 2003 SSC time series represents a minima. As well, river turbidity was measured in East River with an Analite NEP9500 turbidity sensor (± 10.0 NTU over the full range of SSC) logged with a Hobo U12 logger at 30-second intervals in 2005. A comparison of the turbidity time series with the point samples collected from the river demonstrated that the point samples were comparable in most cases, but missed short-lived periods of variability (Figure 2.2). This comparison indicates that point samples likely underestimated the overall variability in SSC and thus suggests that our estimates of SSQ are conservative.
As well, given the stage
measurement problems encountered early in the 2003 East River runoff,
15
seasonal suspended sediment yield was determined to be a gross underestimate in that year. Table 2.1: Differences in SSQ estimates based on spline curves constrained by all available point SSC samples (SSQall) and a reduced number of point SSC samples to reflect the reduced sample interval undertaken in 2003 (SSQ2003). Specific sediment yields (Mg·km-2) are indicated in parentheses. River, Year
SSQall (Mg)
SSQ2003 (Mg)
West 2003
134 (16.8)
n/a
West 2004
413 (51.6)
410 (51.3)
West 2005
63 (7.9)
61 (7.6)
East 2004
433 (37.3)
425 (36.6)
East 2005
108 (9.3)
83 (7.2)
Suspended Sediment . Concentration (mg L)
1000 Point Samples Turbidity Hourly Readings
800 600 400 200 0 Jun 12
Jun 14
Jun 16
Jun 18
Jun 20
Jun 22
Jun 24
Jun 26
Date Figure 2.2: The effect of sample density on estimating total seasonal suspended sediment yield in the East River, 2005. Point samples taken during short-lived high concentration periods induce over-estimates and likewise, point samples taken during short-lived low concentration periods generate under-estimates. The turbidity points shown represent the individual measurement taken on the hour, in conjunction with the manual point sample collected.
16
2.5 Results 2.5.1 Hydrometeorology
Snow surveys conducted prior to runoff in each watershed demonstrated that in seasons with reduced estimated overall snowpack (2003 and 2005), SWE was greater in the West catchment than the East catchment (Table 2.2). However, in 2004 when SWE was substantially higher, snowpack distribution was more uniform across the two catchments. High winds throughout the winter in the High Arctic result in large snowbanks and drifts on the lee slopes and in concave river channels (Yang and Woo, 1999).
Thus, the snow survey results likely
underestimate the total amount of snow in certain terrain classes. In particular, it is highly likely that SWE was underestimated in the river channels as it was not possible to obtain an absolute depth in many portions of the river channel (> 2.5 m probe length). Table 2.2: Estimated snow water equivalence (SWE) and total runoff for each watershed at Cape Bounty compared to the total precipitation prior to each season (total, uncorrected Oct. – May) at Mould Bay, NWT (200 km west). The values from Mould Bay represent minimums as there are months with missing data, as well the precipitation gauge at Mould Bay malfunctioned during early 2005 (Meteorological Service of Canada, pers. Comm. 2005). The total runoff for the East River in 2003 is underestimated due to problems with the stilling well position during initial runoff. West Year
Mould Bay
East
SWE (mm) ΣQ (mm) SWE (mm) ΣQ (mm)
Precipitation (mm)
2003
43
69
20
>24
>89
2004
82
120
41
107
>68
2005
55
81
16
76
–
17
Daily mean air temperature data were highly correlated amongst the Cape Bounty weather stations (r2 = 0.98 – 0.99, n ≥ 70, for all years). Correlation of the Cape Bounty mean daily temperature records with the two closest Meteorological Service of Canada (MSC) stations at Rea Point, Nunavut (r2 = 0.84 – 0.98, n ≥ 70, for all years) and Mould Bay, Northwest Territories (r2 = 0.85 – 0.98, n ≥ 70, for all years; Figure 2.3) was also high. Cumulative melting degree days (MDD) indicate that 2005 was warmer earlier than the other two years studied (Figure 2.4), but was similar to the long-term mean MDD values at the nearby meteorological stations and not anomalously warm in the context of the past 57 years.
In addition, paired t-tests indicated that June 2005 was
significantly warmer than June temperatures in 2003 and 2004 at 95% confidence.
Furthermore, the t-test indicated that there were no significant
differences between June temperatures in 2003 and 2004 at the same confidence level.
18
Mean Daily o Temperature ( C)
12 10 8 6 4 2 0 -2 -4 -6 -8 -10
Mean Daily o Temperature ( C)
12 10 8 6 4 2 0 -2 -4 -6 -8 -10
Mean Daily o Temperature ( C)
12 10 8 6 4 2 0 -2 -4 -6 -8 -10
2003
Rea Point Mould Bay Cape Bounty
2004
Rea Point Mould Bay Cape Bounty
2005
06/01
Rea Point Mould Bay Cape Bounty 06/06
06/11
06/16
06/21
06/26
07/01
07/06
07/11
07/16
07/21
07/26
07/31
Date
Figure 2.3: Daily mean temperature at Rea Point, Mould Bay and Cape Bounty during June and July for the three years of this study.
19
Melting Degree Days
180 160 140 120 100 80 60 40 20 0
Melting Degree Days Melting Degree Days
180 160 140 120 100 80 60 40 20 0 180 160 140 120 100 80 60 40 20 0 06/01
2003
Rea Point
Rea Point Mean
Mould Bay
Mould Bay Mean
July 31
Cape Bounty
June 30 June 15
2004
Rea Point
Rea Point Mean
Mould Bay
Mould Bay Mean
July 31
Cape Bounty
June 30
June 15
2005
July 31
Rea Point Mean
Rea Point
Mould Bay Mean
Mould Bay Cape Bounty
June 30 June 15 06/06
06/11
06/16
06/21
06/26
07/01
07/06
07/11
07/16
07/21
07/26
07/31
Date
Figure 2.4: Cumulative melting degree days (MDD) for each season at Cape Bounty and the long-term means determined on June 15, June 30 and July 31 from Rea Point and Mould Bay weather stations. Three reference lines (June 15, June 30 and July 31) show the cumulative thermal energy available prior to that date. The mean of the cumulative MDD at nearby weather stations are shown by triangles (Rea Point) and circles (Mould Bay) on June 15, June 30 and July 31. The means at Rea Point and Mould Bay are based on measurements between 1969–2005 and 1948–2005, respectively.
After initial ponding of meltwater in the streams in early to mid-June, channelized flow was established at the gauging stations within 6-7 days in 2003 and 2004 and in less than 8 hours in 2005. Discharge was characterized by a distinctive diurnal cycle that peaked at approximately 1700-1900h in both rivers. Time to peak runoff was approximately a week in the first two years of the study, 20
and less than 2 days in 2005 (Figure 2.5). The date of initial and peak discharge differed between the rivers by seven days in 2003, likely due to the reduced snowpack in the East River watershed and channel, which required less time to ripen and saturate with meltwater. In the West River, flow was rerouted through a subnival channel after initial channelization in 2003 and 2004, but remained on the snow surface channel in 2005 for the entire season (Lamoureux et al., 2006a). Peak runoff duration and instantaneous peak discharge were similar between the rivers in each respective season (Figure 2.5). Discharge responses due to rainfall events during the summer were minor and short-lived (e.g., Figure 5a, July 28, 2003). A comparison of the total runoff each season suggests that SWE was significantly underestimated by the snow survey network and subsequent surveys were expanded to improve representation (Table 2.2). Although SWE underestimated total runoff, it predicted the relative difference between watersheds and between years. Therefore it appeared reasonable to use these data to relate hydroclimatological controls on seasonal sediment discharge at the Cape Bounty study site. Estimates of seasonal snow accumulation from regional weather stations were not comparable due to the unrepresentativeness of such data (Woo et al., 1999; Yang and Woo, 1999). Furthermore, precipitation data were not available for Rea Point, and missing data and instrument malfunction (2004-5)
at
Mould
Bay
precluded
comparable
(Meteorological Service of Canada, pers. comm. 2005). 21
data
from
the
station
Hourly Air o Temperature ( C)
Hourly SSC (mg/L)
-8
-4
0
4
0 Jun Jun Jun Jul Jul Jul Jul Jul Jul 3 0 2 1 2 1 20 25 30 0 5 5 0 0 5 Date
0
4
6
8
10
12
2
SSQ = 134 Mg
1.8 1.5 1.2 0.9 0.6 0.3 0.0
12
8
4
100
200
300
400
500
0
500
1000
1500
2000
Total Q = 69 mm
Figure 2.5a, figure caption follows
SSQ (Mg)
0
3
5
Hourly 3 Discharge (m /s)
Rainfall (mm) Q (x 10 m )
Hourly Air o Temperature ( C) Hourly SSC (mg/L) 0
100
200
300
400
500
6000 5000 4000 3500 3000 2500 2000 1500 1000 500 0
-8
-4
0
4
8
12
16
Hourly 3 Discharge (m /s) SSQ (Mg)
SSQ = 413 Mg (410 Mg)
Total Q = 120 mm
2
4
6
8
10
12
1.8 1.5 1.2 0.9 0.6 0.3 0.0
12
8
4
0
0 Jun Jun Jun Jul Jul Jul Jul Jul Jul 1 1 3 2 2 0 20 25 30 0 5 0 0 5 5 Date
2004
-8
-4
0
4
8
12
16
0
100
200
300
400
500
0
500
1000
1500
2000
Hourly
8
2003
3
5
Hourly SSC (mg/L)
SSQ (Mg)
Rainfall (mm) Q (x 10 m )
Hourly Air o Temperature ( C) Discharge (m3/s)
12
No rainfall
Jun
Ju Ju Ju Ju Ju 01 n 06 n 11 n 16 n 21 n 26 Date
SSQ = 63 Mg (61 Mg)
Total Q = 81 mm
2005
0
2
4
6
8
10
12
1.8 1.5 1.2 0.9 0.6 0.3 0.0
3
5
16
Q (x 10 m )
22
Date
12
8
4
100 0
4 2 0
200 100 0
Date
Ju Ju Ju Ju Ju Ju Ju Ju 20 n 25 n 30 l 05 l 10 l 15 l 20 l 25 l 30
200
6
300
400
300
500
0
500
8
12
14
1000
1500
2000
2500
3000
-8
-4
0
4
8
12
16
400
SSQ = 433 Mg (425 Mg)
Total Q = 107 mm
1.8 1.5 1.2 0.9 0.6 0.3 0.0
12
8
4
0
10
Jun
2004
500
0
500
1000
1500
2000
2500
3000
-8
-4
0
4
8
12
16 o Air Temperature ( C)
0
1.8 1.5 1.2 0.9 0.6 0.3 0.0
Jun Jun Jun Jul Jul Jul Jul Jul Jul 18 23 28 03 08 13 18 23 28
Point Discharge Measurments
Figure 2.5b, Figure caption follows
0
500
1000
1500
2000
2500
3000
-4
0
4
8
12
2003
Hourly SSC (mg/L)
o Air Temperature ( C)
Hourly SSC (mg/L)
Rainfall (mm) o Air Temperature ( C) Discharge (m3/s) SSQ (Mg)
Discharge (m3/s) Hourly SSC (mg/L) SSQ (Mg)
16
Jun
01
Total Q = 67 mm
No Rainfall
Jun
06
Jun
Date
11
Jun
16
Jun
21
Jun
26
SSQ = 108 Mg (83 Mg)
2005
0
2
4
6
8
10
12
14
1.8 1.5 1.2 0.9 0.6 0.3 0.0
3
Rainfall (mm) 3
Q (x 10 m ) 5
Discharge (m3/s) Q (x 10 m ) 5
23
Figure 2.5: Hourly hydrometeorological summaries for (a) West and (b) East River catchments. Note that the time period shown is different for each year. Values in parentheses on the sedigraph are the total SSQ estimates based on a reduced sample set in order to be comparable to the dataset collected in 2003 (see text for description). The hydrograph at the beginning of 2003 in East River is unavailable because the channel where the gauging station was located was not free of snow at this time. The bottom panel for each year shows the cumulative discharge (m3) and cumulative suspended sediment yield (Mg).
2.5.2 Sediment Delivery
Suspended sediment concentration (SSC) reached seasonal maximums after peak runoff in both rivers in all cases except in the East River 2005, when peak SSC occurred prior to peak discharge (Figure 2.5). In general, SSC remained low prior to peak discharge. However, as runoff and channelization progressed, access to sediments and SSC increased. Maximum SSC varied considerably each year, but was substantially higher in 2004 (5526 mg·l, West River). During the same season, higher SSC was maintained over a longer duration in both rivers compared to 2003 and 2005. In 2003 and 2004, the mean SSC after peak discharge was substantially larger than the mean SSC during the same periods in 2005 (Figure 2.5). In 2003 and 2004 the majority of suspended sediment was transferred in less than one week (Figure 2.5).
After the nival peak, discharge and SSC
decreased substantially, and resulted in relatively minimal suspended sediment discharge (SSQ). In 2005, SSQ (Figure 2.5) was more uniform over the entire runoff season compared to the previous two years. Although the 2005 season was comparatively short due to reduced snowpack and warm conditions, further 24
appreciable snowmelt-sourced discharge was unlikely when observations ceased.
2.6 Discussion 2.6.1 Hydroclimate controls over seasonal runoff
Previous studies in the arctic have pointed to the short-lived, intense nival peak as the most significant period for suspended sediment transport (Lewkowicz and Wolfe, 1994; Hardy, 1996; Braun et al., 2000; Priesnitz and Schunke, 2002; Beylich and Gintz, 2004; Forbes and Lamoureux, 2005). Hence, it is important to consider the hydroclimatic controls that contribute to nival runoff.
Seasonal
discharge in the Cape Bounty rivers was generated and sustained primarily by snowmelt over three seasons (Figure 2.6). Runoff intensity was proportionate to initial SWE and the rate at which snowmelt water was produced.
The clearest
indication of the dominant control of SWE over discharge was the response of both rivers in 2004, a year with relatively high SWE and low available melt energy (Figures 2.5, 2.6). By comparison, reduced SWE in both 2003 and 2005 resulted in substantially lower peak discharge, duration of peak runoff and sediment delivery, and lower total runoff and suspended sediment yield (Figure 2.6). Total runoff is the net of winter snowfall, and losses due to ablation and evaporation and infiltration and resultant soil storage. Losses due to ablation and evaporation will be minimal in cool springs due to limited available thermal 25
energy (Woo and Sauriol, 1980; Woo and Young, 1997).
In warm springs,
ablation losses may be greater, and the snowpack may become fragmented due to rapid melting. In 2005 snowmelt began three weeks earlier than the previous two springs (Figure 2.4). Combined with reduced SWE, the early warm spring in 2005 produced an accelerated melt period and rapid channelization.
The
snowpack on slopes and uplands was fragmented and isolated earlier which resulted in a reduction of the runoff contribution area. The reduced connectivity of the fragmented slope snowpack further delayed meltwater runoff from reaching the channel, and introduced greater potential for infiltration into newly thawed soil and increased flow resistance. In 2003 and 2004, reduced available melt energy slowed snow cover losses, particularly in areas with thin snowpack, resulting in more extensive snow cover through the melt period. Thus, conditions in these years maintained a larger contributing area to flow throughout the peak snowmelt period that sustained high discharge for longer.
26
(b)
4
60
2003 SWE
40
40 West 2003 West 2004 West 2005
20 0 0
20
40
60
80
20
0 100 120 140
40
100
East 2004 East 2005
80 60
20
2005 SWE
40
30
10
20 0 0
20
40
60
80
0 100
Estimated SWE (mm)
Cumulative Discharge 4 (x10 m3)
2004 SWE
120
200 100
80 60 40 20
0 20
40
60
80
0 100 120 140
Cumulative Melting Degree Days 500
50 2004 SWE
400
40 East 2004 East 2005
300 200
30 20
2005 SWE
100
10
0 0
Cumulative Melting Degree Days
West 2003 West 2004 West 2005
2005 SWE 2003 SWE
(d) 50
140
300
0
Cumulative Melting Degree Days
(c)
400
100 2004 SWE
Estimated SWE (mm)
2005 SWE
60
80
Cumulative Suspended Sediment Yield (Mg)
80
500
20
40
60
80
Estimated SWE (mm)
2004 SWE
Cumulative Suspended Sediment Yield (Mg)
100
Estimated SWE (mm)
100
(x10 m3)
Cumulative Discharge
(a)
0 100
Cumulative Melting Degree Days
Figure 2.6: Cumulative discharge (a and c) and suspended sediment yield (b and d) compared to cumulative melting degree days (MDD) for West (a and b) and East Rivers (c and d). Estimated SWE for each catchment and year it represents is indicated by horizontal dashed lines.
In addition to SWE magnitude, runoff intensity also depends on the rate of snowmelt water production (Woo, 1983). A season with reduced thermal energy inputs can generate a more intense runoff due to meltwater stored within snowbanks and the snowpack. Woo and Sauriol (1980) observed that cooler springs delayed peak runoff due to ponded meltwater, and consequently increased runoff intensity in rivers near Resolute.
Additionally, they also
observed that cooler springs reduced overall ablation losses when accompanied by reduced solar radiation due to increased cloud cover (Woo and Sauriol, 1980; Woo and Young, 1997). In 2003 and 2004, the snowmelt period at Cape Bounty 27
was prolonged due to cool conditions (Figure 2.4). Meltwater produced in early spring was temporarily stored within the snowpack and in thick snow banks within the channels for up to a week. In several instances ponding was observed behind deep, near-saturated channel snow banks which increased the potential meltwater runoff in the catchments. A prolonged period of ponding occurred in West River in 2003 and 2004 (seven days) but once the channel was established, runoff was intense. However, in 2005, ponding occurred for only eight hours due to the small volume of snow and rapid snowpack melting during the warm spring. Thus, runoff intensity was reduced in 2005 due to the lack of water storage and limited meltwater production. The secondary, but important links between runoff intensity and thermal conditions are demonstrated through a comparison between 2003 and 2005. Although 2003 and 2005 had similar SWE estimates and total discharge, the delayed release of meltwater in 2003 generated more intense runoff compared to 2005. Melt energy available in 2003 was reduced compared to 2005 (estimated by total melting degree days; Figure 2.4) and led to meltwater storage within the snowpack and seven days of ponding in the channels. In this respect, 2003 was quite similar to 2004 and both years exhibited increased runoff intensity. The observations from Cape Bounty are consistent with and contribute to a growing number of studies that indicate that the primary control over nival runoff is through catchment snowpack. The increase in total seasonal discharge associated with larger snowpacks is typically clear (e.g., Lewkowicz and Wolfe, 28
1994; Forbes and Lamoureux, 2005). However, the results from both this study and previous work suggest that increased spring snowpack also appears to lengthen the duration of high discharge during the spring (e.g., Forbes and Lamoureux, 2005). These conditions are mediated by available melt energy and in many instances, daily discharge is significantly correlated with temperature (Hardy, 1996; Forbes and Lamoureux, 2005).
However, these relationships
become more complex or weaken as snowpack is progressively exhausted (Forbes and Lamoureux, 2005).
Hence, while the relationship between melt
energy and daily discharge may be important for discharge generation during the nival peak, seasonal discharge appears primarily governed by the amount of snow available. Melt energy and snowpack distribution contribute as secondary factors and are important in distinguishing between years with similar SWE (e.g., 2003 and 2005). It is of particular note that increased discharge during the nival peak may not necessarily result in higher instantaneous discharge. Rather, the period of high discharge may be prolonged for several days and result in substantially higher total discharge (Forbes and Lamoureux, 2005).
2.6.2 Hydroclimate controls on seasonal sediment delivery
Total runoff generated by snowmelt each spring was the most important hydroclimatic factor controlling seasonal sediment delivery at Cape Bounty. Runoff intensity appeared to be a secondary condition controlling seasonal suspended sediment yield. Total seasonal sediment delivery was greatest in 2004 (Figure 2.5; Table 2.1; West 413 Mg, East 433 Mg) in response to the 29
largest spring snowpack, total runoff and runoff intensity. Additionally, increased runoff resulted in a disproportionately larger increase in SSQ.
Comparison
between 2003 and 2004 reveals that 2004 runoff was nearly double, but SSQ increased by nearly four times. In 2003 and 2005 when SWE and total runoff were similar (Figure 2.5) the corresponding seasonal SSQ was dissimilar because each watershed responded differently. The disproportionate response between the three seasons studied is likely reflected in the differences in runoff intensity and possibly interannual sediment supply. In the West River 2003 and 2005, SWE and total runoff were similar, but SSQ was substantially reduced in 2005. The major difference between the two seasons was that runoff was more intense in 2003 because snowmelt runoff was prolonged due to reduced thermal energy (Figure 2.4).
In 2005, runoff was
characterized by reduced peak instantaneous discharge and SSC and therefore the stream competence was reduced (Figure 2.5a). Furthermore, cumulative SSQ shows a gradual transfer of sediment in 2005 rather than rapid transfer over a short period of time as observed in the preceding two years (Figure 2.5a, 2005 bottom panel).
The East River responded similarly, with gradual sediment
transfer in 2005 compared to rapid sediment transfer over a few days in 2004 (Figure 2.5b, bottom panel). These results demonstrate that seasonal SSQ does not proportionately respond to total runoff and likely reflects the duration of maximum instanteous discharge (intense runoff) and SSC during the season (Forbes and Lamoureux, 30
2005). In 2003 and 2004, the majority of suspended sediment transfer occurred over a short period of time and reflects the importance of flow competence and sediment availability during this period.
These results are similar to the
responses reported in other arctic river systems. For example, in a study of two watersheds on Ellesmere Island, Nunavut, 86 – 99% of the seasonal suspended sediment load was transported during the main melt period (Lewkowicz and Wolfe, 1994).
Additionally, peak instantaneous discharge was substantially
higher in the year with greater SWE (~15 m3·s-1 (SWE 118 mm) and 3.8 m3·s-1 (SWE 43 mm); Lewkowicz and Wolfe, 1994), which in part, reflects the differences in SWE between years and a delayed spring in the former (Woo et al., 1991). In a multi-year study of two creeks in the Richardson Mountains, northern Yukon, the greatest sediment delivery occurred at the transition into the late nival flood phase, where 99% of the annual suspended load was delivered during the five-day snowmelt period (Priesnitz and Schunke, 2002). Similarly, Forbes and Lamoureux (2005) observed that the only time three middle arctic rivers carried appreciable sediment was during the brief period (several days) of maximum discharge and noted that increased catchment SWE sustained the period of high discharge and effectively increased seasonal SSQ. Their results showed that a SWE increase of approximately 1.7 times corresponded to 3.5 times greater total SSQ in the Lord Lindsay River (Forbes and Lamoureux, 2005).
31
Analysis of hydroclimatic controls on sediment delivery by Hardy (1996) indicated that thermal indices could reasonably estimate total seasonal SSQ from a mountainous watershed on northern Ellesmere Island, although SWE information was not included in the study. Similar analysis at Cape Bounty with daily melting degree days (MDD), total daily discharge and total daily SSQ (Table 2.3) demonstrate that the strongest correlations were during initial sediment transfer only. Even though the strongest correlations were observed early in the season, the relationship was not consistent each year, or between the rivers. In the warmest season (2005) at Cape Bounty, discharge and suspended sediment yield were poorly correlated with daily MDD (Table 2.3) unlike the previous years when suspended sediment yield was more strongly correlated with daily MDD in the early season.
This suggests that despite warmer conditions, runoff from
snowmelt was the dominating control in sediment yield and cooler conditions led to more intense runoff and sediment transfer. For example, 2005 was warmer and correlations between daily suspended sediment yield in the West River and temperatures suggest that the warmer conditions in 2005 did not have a positive influence on the overall sediment yield. Furthermore, in each river, most of the suspended sediment flux occurred prior to major accumulation of MDD. This suggests that daily MDD may be a poor predictor of seasonal discharge and total sediment yield in a given year, especially where there is a large spring snowpack at Cape Bounty.
32
Table 2.3: Regression coefficients (r) for daily discharge, suspended sediment yield and melting degree days (MDD) for each river during the periods of highest and lowest daily runoff and sediment transfer rates. The 2003 East Season is not reported due to the stilling well problems at the beginning of the season. The 2005 season was not separated into high and low rate periods due to the short record available. West River
West River
West River
East River
East River
East River
High Rate
Low Rate
Total Season
High Rate
Low Rate
Total Season
Year Q vs MDD (n)
2003 2004
SSQ vs MDD (n)
Q vs MDD (n)
SSQ vs MDD (n)
Q vs MDD (n)
SSQ vs MDD (n)
Q vs MDD (n)
SSQ vs MDD (n)
Q vs MDD (n)
SSQ vs MDD (n)
0.47
0.98
-0.19
-0.18
-0.09
-0.14
(7)
(4)
(29)
(32)
(36)
(36)
0.11
0.79
-0.03
-0.06
0.14
0.42
-0.10
0.73
0.03
0.29
(15)
(9)
(22)
(28)
(14)
(11)
(23)
(26)
2005
Q vs MDD (n)
SSQ vs MDD (n)
n/a
n/a
0.08
0.42
(37)
(37)
(37)
(37)
-0.43
-0.15
0.06
0.45
(15)
(15)
(18)
(17)
Despite the dominance of snowpack controls over total SSQ, thermal conditions likely played an indirect role in suspended sediment delivery at Cape Bounty through pre-runoff snow ablation. In 2005, conditions were substantially warmer than the previous two years and caused rapid snowpack fragmentation and melt that reduced runoff and limited sedimentation erosion from many firstorder channels. By contrast, 2003 had a similar SWE but the snowpack was substantially less fragmented.
Increased connectivity of first-order sediment
supplies may in part explain the higher sediment yields in 2003 compared to 2005.
33
Finally, in 2003 and 2004, the West River tunneled under thick channel snowpacks to access sediment on the river bed. In 2005, the river did not tunnel beneath the snowpack and thus the river had reduced access to sediment supplies through the peak runoff period (Lamoureux et al., 2006a).
Similar
tunneling was not apparent in the East River in any of the years studied, hence it is difficult to know the extent to which isolation from the channel bed could have affected the 2005 sediment yield (e.g., Woo and Sauriol, 1980; 1981). In addition to snowpack meltwater production controls over total runoff and runoff intensity in a season, these results suggest that a third factor may influence seasonal sediment yield at Cape Bounty. Despite similar SWE and total runoff, total SSQ in 2005 West River was less than half of the 2003 yield. A key difference between the years was the lower overall SSC and reduced instantaneous peak discharge in both rivers during 2005.
It is possible that
reduced yields in 2005 were caused by some degree of reduced sediment availability; essentially a form of interseasonal sediment hysteresis that may have been caused by sediment exhaustion due to high sediment yields in 2004. As well, observations suggested that some sediment supplies, available early in the season during 2003 and 2004 and resulted in significant deposits of sediment on channel snowpack, were unavailable in 2005 (Lamoureux et al., 2006a). If sediment availability was reduced in 2005, the observed differences between the West and East Rivers (Table 2.3) suggest that the West River was more affected by interannual sediment exhaustion. The apparent difference in 34
sediment supply between the watersheds may be explained by differences in watershed geomorphology.
In general, the West watershed has narrower
channels and steeper slopes compared to the broader valley in the East watershed. This potentially leads to more snow being trapped in depressions and gullies in the West catchment. As well, snow cover was generally patchier in the East catchment compared to the West catchment, likely due to prevailing winter winds redistributing snow.
Furthermore, Lamoureux et al. (2006a)
demonstrated that ponding in early spring can abandon a substantial amount of sediment on multi-year channel snow banks in the West River, nearly 17% of the annual sediment yield in 2003.
Similar ponding in the East River was not
observed and potentially is less likely due to the broader channels.
This
suggests that sediment source and channel storage mechanisms are more complex in the West River watershed. Although these results suggest that the West River is more sensitive to interannual sediment supply variations than the East River, the available data are not sufficient to conclusively demonstrate the extent to which hysteresis actually occurred and how consistent this differential sensitivity would be with different snowpack and hydroclimatic conditions.
2.6.3 Sensitivity of sediment yield to climate variability in high arctic watersheds
The results from this study suggest that sediment transfer is most sensitive to runoff conditions during the nival freshet which are primarily controlled by catchment SWE, and to a lesser extent, melt energy and snow cover distribution. 35
Comparison of adjacent catchments with similar underlying bedrock, surficial materials and vegetation cover suggests that interannual sediment yield variations are also likely subject to localized, potentially important geomorphic controls. Therefore, climate model results that predict future increased winter precipitation have important implications on sediment transfer in nival-dominated Arctic river systems. Syvitski (2002) modeled sediment loads with respect to temperature and discharge increases and concluded that 2ºC warming would increase sediment loads by 22%. This was partially due to greater sediment availability due to increased active layer thickness (Woo et al., 1992; ACIA, 2005), but also due to larger snowpacks and nival freshets (Syvitski, 2002). Results from Cape Bounty are in agreement with these results, although the short record prevents analysis of the role of changing active layer thickness on sediment yield.
The
disproportionate increase in sediment yield between 2003 and 2004 in response to greater SWE suggests greater yield sensitivity than the model results, although the modeling was based on much larger watersheds (Syvitski, 2002). Moreover, the apparent sensitivity may reflect local sediment availability characteristics, which vary widely across the Canadian Arctic (Lewkowicz and Wolfe, 1994; Lamoureux, 2000). For example, Forbes and Lamoureux (2005) also found disproportionate responses in watersheds approximately 100 times larger than the Cape Bounty watersheds, suggesting that the response observed at Cape Bounty may scale up in some cases. However, Forbes and Lamoureux 36
(2005) also reported low yields, so it is difficult to determine if the limitations are due to scale issues or sediment supply. Although temperature was not shown to be a primary control over seasonal sediment transfer at Cape Bounty, the impact of warmer temperatures in the future may influence sediment supply in the catchment through permafrost degradation and surface disruption. The three years observed at Cape Bounty are insufficient to observe any changes in sediment supply due to the impact of warmer summers and perhaps increased permafrost degradation.
However,
sedimentary records from lakes and ponds have been used to estimate past sediment yield (e.g., Lamoureux, 2002; Verstraeten and Poesen, 2002). Lakes and ponds that receive clastic sedimentary inputs can serve as natural archives of seasonal sediment runoff from the catchment. Examination of the sedimentary records from the downstream lakes at Cape Bounty is underway to quantify past sedimentation patterns and provide additional means to evaluate sediment yield departures and interannual hysteresis.
2.6.4 Interpreting hydroclimatic variability from downstream sedimentary records
Examination of the hydrometeorological measurements at Cape Bounty with concurrent regional observations suggests none of the years monitored were extremes with respect to temperature; thus, our observations may be considered typical of these watersheds for the past 57 years (Figure 2.7). Interpretation of the precipitation records over the region is problematic as wind re-distribution of 37
snow is significant in the Arctic, especially in years with low snow-cover (Yang and Woo, 1999). As already discussed, the total precipitation and snowpack at Cape Bounty do not correlate with precipitation from weather stations at Mould Bay or Resolute which are located at sea level on the coast compared to the snow survey results from Cape Bounty, which were carried out over a range of elevations (20–120 m a.s.l.). The multi-season study of sediment transfer at Cape Bounty has important implications for the interpretation of the sedimentary records in lakes subject to similar watershed processes. Catchment studies have been used to quantify the relationships
between
hydrometeorological
conditions
and
sedimentation
processes in order to infer the climate signal preserved within the sedimentary record (e.g., Hardy et al., 1996; Gilbert and Butler, 2004). There are few studies in the Arctic that pair paleoclimate reconstructions from a site with a catchment process study, although there has been success in statistically interpreting the paleoclimatic record from lake sediments without an associated process study (e.g., Hughen et al., 2000; Francus et al., 2002; Hambley and Lamoureux, 2006). In a pioneering study, Hardy et al. (1996) concluded that early summer temperature was significantly correlated to annual sedimentary layer thickness in a lake with a partially glacierized drainage basin on northern Ellesmere Island based on a process study completed at the site (Hardy, 1996). In addition to warmer temperatures, runoff was an order of magnitude larger in the warmer spring and subsequently generated a greater sediment yield (Hardy, 1996). 38
Although there were no SWE data available and the site likely receives some glacial-meltwater inputs each summer, the study established a strong climatological (thermal) link with sediment delivery processes in the Arctic. However, as in this study, there is a strong relationship between total discharge and seasonal sediment delivery at Lake C2. In a recent study in the Canadian Middle Arctic, Lamoureux et al. (2006b) interpret the annually laminated sediments in Sanagak Lake, Boothia Peninsula, Nunavut as a record of spring discharge controlled by SWE based on two years of process studies that included characterization of SWE (Forbes and Lamoureux, 2005). The Boothia study characterized the relationship between hydrological process and sediment deposition and demonstrated the potential to explore the linkages between climate and hydrology through laminated lake sediment records.
39
o Mean Temperature ( C)
(a) 4
Mean June Temperature
2 0 -2
Mould Bay Rea Point Cape Bounty
-4 -6 1950
o Mean Temperature ( C)
(b)
1955
1960
1965
1970
1975
1980
1985
1990
1995
2000
2005
Year AD 8
Mean July Temperature
6 4
Mould Bay Rea Point Cape Bounty
2 0 1950
1955
1960
1965
1970
1975
1980
1985
1990
1995
2000
2005
Year AD
Figure 2.7: Mean monthly (a) June, and (b) July air-temperature records from Mould Bay and Rea Point weather stations. Note that an equipment change (to automated measurement) at Mould Bay resulted in gap in the record.
Our results demonstrate that seasonal sediment transfer in nival watersheds is dependent primarily on SWE and total runoff, and secondly, on runoff intensity. Therefore, the sedimentary record in the downstream lakes may also reflect nival melt magnitude and intensity.
However, temperature can
indirectly mediate suspended sediment transfer through melt generation and connectivity of runoff and sediment supply sources.
Furthermore, warmer
temperatures may increase sediment availability through increased permafrost degradation and potential sediment availability over longer time scales. 40
The indications of possible inter-annual sediment yield hysteresis at Cape Bounty raise a critical issue for the interpretation of sedimentary records as hydroclimatic proxies.
Multi-year sediment yield exhaustion could conceivably
alter the sedimentary record by dampening sediment accumulation following high yield years like 2004. To date, little work has been carried out to investigate this issue.
A study of a varve record from a nivally-dominated system identified
sediment supply effects that lasted multiple decades (Lamoureux, 2002). Annual mass accumulation during the past 487 years revealed evidence for sustained high sediment yields for up to 17 years after a year with an exceptional yield (Lamoureux, 2002). The impact of this type of sediment availability is important to consider as part of the hydroclimatic interpretation of the sedimentary record, and varies substantially between lake systems. For example, in a study from Sophia Lake, Cornwallis Island, Nunavut, sediment supply was limited to thin, discontinuous surficial deposits and resistant carbonate bedrock. In this case, the recurrence of high sediment yields after a large event was considered unlikely (Braun et al., 2000). Further work to explore these effects is clearly warranted, given the growing number of paleoenvironmental records derived from sedimentary records.
2.7 Conclusions
Seasonal suspended sediment yield from two high arctic catchments was controlled by SWE through total runoff and runoff intensity in a given season. Interannual seasonal suspended sediment yield increased disproportionately in 41
response to higher total discharge through prolonged high instantaneous discharge and SSC. Variable snowcover altered the production and intensity of meltwater runoff, and influenced the sediment yield by isolating runoff from sediment sources, particularly in the channel. Furthermore, comparison of the two catchments suggests that increased SWE, and resultant large runoff and suspended sediment yield in 2004 may have exhausted sediment supplies and reduced yields in the subsequent year. Each watershed exhibited a different degree of inferred sediment exhaustion, indicating the importance of watershedspecific conditions. Given the observed response to different snow years, it is likely that sediment yields in this environment will increase in response to increased winter precipitation predicted by current models.
In addition, although increased
temperatures play a secondary role in controlling seasonal sediment yield in this study, it is likely that warmer temperatures will also increase permafrost degradation and potentially increase sediment supplies. The three years studied were not anomalous with respect to temperature records for the last 57 years from weather stations in the region. Therefore, the results reported here appear representative of the typical sediment transfer conditions in these streams for that period. However, longer records of seasonal sediment transfer processes are still needed to evaluate interannual sediment hysteresis and further elaborate the likely responses of arctic watersheds to projected climate change. Finally, these results demonstrate the need to evaluate long-term sediment delivery 42
processes and to carefully consider watershed processes prior to the interpretation of downstream sedimentary records.
43
Chapter 3 Inflow and lake controls on short-term mass accumulation and particle size in a High Arctic lake: implications for interpreting varved lacustrine sedimentary records In Press, Journal of Paleolimnology Authors: Jaclyn M.H. Cockburn Scott F. Lamoureux
Keywords: Suspended sediment discharge; deposition; turbidity; grain size; paleoclimate; laminae
3.1 Abstract:
Sedimentary processes monitored in a lake with varved sediments in the Canadian High Arctic through three melt seasons revealed that seasonal sediment deposition rates were highly dependent on short-lived inflow events driven by high suspended sediment concentrations that varied with runoff intensity.
Our results illustrate that in accordance with suspended sediment
discharge into the lake, the rate of sediment accumulation changed over short distances down-lake, in a given year. This result indicates that there is a rate and accumulation dependence on short-lived, intense inflow conditions.
In
addition, there was strong evidence for substantial decoupling between deposition rate and mean grain size of sedimentary deposits. These results have 44
important implications for paleoclimatic interpretation of annually laminated sedimentary records from dynamic lake environments and suggest that grain size measures may not be representative proxies of inflow competence. Grain size indices based on a measure of the coarser fraction, rather than the bulk sediment, may be more appropriate to use as a link between contemporary runoff processes and sedimentary characteristics.
3.2 Introduction
Several
multi-year
studies
have
examined
the
relationship
between
hydroclimatologic behaviour and sediment delivery to arctic lakes (e.g. Retelle and Child 1996; Braun et al. 2000; Lewis et al. 2002) in an effort to quantitatively understand the factors that control the formation of the lacustrine sedimentary record. In addition to these studies, physical limnologic studies elsewhere have focused on contemporary processes that control seasonal and/or annual sediment deposition in lakes and have assisted in the paleolimnologic interpretation of sedimentary records (e.g. Gilbert 1975; Ross and Gilbert 1999; Gilbert and Butler 2004). In cases where sedimentary deposits are annually laminated, other studies have described spatial variability in sediment deposition through time and through multiple sediment core studies in the High Arctic (Lamoureux 1999; 2000; 2002) and in other alpine regions (Smith 1978; Leonard 1997; Menounos et al. 2006; Schiefer 2006a, b). Although these studies have emphasized seasonal mass accumulation, recent work has suggested grain size may be a potentially useful sedimentary parameter to evaluate past hydroclimatic 45
conditions from the sedimentary record (Francus et al. 2002). However, to date, few field studies have demonstrated that sedimentary grain size relates to inflow characteristics (e.g., Sundborg and Calles 2001). Several recent studies (e.g. Lamoureux 1999; Hodder et al. 2007) have identified the need to examine the direct and indirect links between sedimentary proxy records of past hydrometeorologic behaviour. In this paper, we report results from a study that investigated the detailed sediment delivery characteristics and deposition patterns in a High Arctic lake located at Cape Bounty, Melville Island, Nunavut, Canada, for three melt seasons (2003, 2004 and 2005).
In addition to the documented seasonal relationship between
hydroclimatological processes and physical sedimentation in West Lake, the character (texture) of the seasonal deposits was evaluated to determine if particle size could be used as a representative hydroclimatic proxy indicator. In an effort to understand the contemporary hydroclimatic processes that influence sediment deposition, we hope to further elucidate the environmental signal preserved in varved and other sedimentary sequences.
3.3 Study Site
Melville Island is located in the western Canadian Arctic Archipelago (Figure 3.1). At Cape Bounty (74º55’N, 109º35’W, Figure 3.1), there are several, freshwater coastal lakes fed by small rivers draining non-glaciated watersheds without
46
Figure 3.1: (a) Cape Bounty, Melville Island, Nunavut, and locations of meteorological and hydrological stations. Inset shows location of Melville Island in the Canadian Arctic Archipelago. (b) West Lake bathymetry and limnological monitoring sites.
47
present-day glaciers. The landscape is characterized by gentle hills and incised plateaux mantled with Late Wisconsinan glacial and Holocene marine sediments (Hodgson and Vincent 1984). Vegetation cover in this continuous permafrost region is classified as graminoid tundra, which is dominated by patchy sedges and other prostrate dwarf-shrub and ford tundra species (Walker et al. 2005). West Lake (unofficial name) is located ~5 m above sea level and has a maximum depth of 34 m (Figure 3.1). Nearby weather stations at Mould Bay, Northwest Territories (250 km west) and Rea Point, Nunavut (100 km northeast) have relatively long summer temperature records that extend to 1948 and 1969, respectively. Mean June and July temperatures from these stations are similar and demonstrate that initial melt generally begins in June and lasts until mid- to late August. Mean June and July temperatures were 0.0ºC and 3.8ºC at Mould Bay (1948-2006) and were 0.1ºC and 4.0ºC, respectively, at Rea Point (1969-2006).
The West Lake
watershed has a maximum elevation of 126 m, which is substantially higher than the low elevation stations at Mould Bay and Rea Point, but previous work suggests that these stations and Cape Bounty experience broadly similar meteorological conditions (Cockburn and Lamoureux, 2008a). Runoff is dominated by snow melt which typically reaches peak discharge during a one-week period between mid to late June.
Maximum suspended
sediment in the river typically occurs early in the runoff season and is associated with peak discharge (McDonald 2007). Snowpack was found to be the dominant 48
control on seasonal river runoff and suspended sediment discharge in the West River and the adjacent watershed (Cockburn and Lamoureux 2008a). The short period of intense runoff is the main control over seasonal suspended sediment transfer (Cockburn and Lamoureux, 2008a) and is consistent with similar studies of nival Arctic watersheds (e.g. Lewkowicz and Wolfe 1994; Braun et al. 2000; Forbes and Lamoureux 2005). Infrequent, low-intensity (less than 12 mm/d) rainfall occurred through the summer months in 2003 and 2004 and generated limited runoff responses, while no events were observed in the 2005 melt season.
3.4 Methods 3.4.1 Hydrometeorology
Hydrometric monitoring at Cape Bounty began in early June 2003 and continued each melt season thereafter. Snow surveys were completed prior to runoff each spring in order to estimate snow water equivalence (SWE) and potential runoff for each season. The snow survey network designed in 2003 was expanded in 2004 and again in 2005 as familiarity with the area increased (Cockburn and Lamoureux, 2008a).
Transects were established across the watershed in
different landscape units. Each 100 m-long transect was comprised of 11 depth measurements and at least one density measurement. Transect SWE estimates were averaged for each terrain unit and catchment SWE was determined from a terrain-weighted mean (Yang and Woo, 1999). Weather stations were 49
established in two locations in the watershed (Figure 3.1a). Precipitation was recorded with Davis Industrial gauges (0.2 mm resolution, 4% accuracy) recorded with an Onset Hobo event logger or Unidata Prologger.
Air
temperature was recorded at 10-minute intervals with a shielded Onset Hobo H8 (0.4ºC accuracy, West Met) or Onset HoboPro loggers (0.2ºC accuracy, Main Met Station) at 1.5 m above the ground. Systematic wind, incoming solar and net radiation, and relative humidity measurements were also recorded, but these data were not used for this study. The river gauging station, established upstream of West Lake, recorded water stage with a Sensym SCX vented differential pressure transducer recorded at 10-minute intervals with an Onset Hobo H8 (±2 mm) in 2003, and Omega CPLevel101 (±0.2%, 0.5 mm) pressure transducer logger with an Omega CPPRTEMP101 (±0.4% atmospheric pressure) logger for barometric compensation in 2004 and 2005. Water temperature was measured with a Campbell Scientific 107-L water temperature sensor (±0.2ºC) logged with a CR10 logger in 2003 and with the Omega pressure logger (±0.2% water temperature) in 2004 and 2005. Stream rating was carried out at the gauging station using either a Columbia current meter (±4%) or General Oceanics Flowmeter (±1%) at regular intervals during the runoff period. Point suspended sediment samples were collected with a DH48 integrated water sampler three times daily in 2003, and hourly through the peak runoff and bi-hourly during the recession period in 2004 and 2005. Volumetric suspended sediment samples were filtered in the field on tared 0.45 50
µm Whatman cellulose acetate filters (2003) and Osmotics 1.0 µm glass fibre filters (2004, 2005) and returned to the laboratory to be dried at 50ºC and weighed to calculate suspended sediment concentrations through the season. The change in filters after 2003 was required to increase sample processing capacity and comparative tests revealed minimal impacts on resultant suspended sediment concentrations.
3.4.2 Limnology
Perennial lake ice was present during all seasons and provided a platform for limnological deployments. Bathymetry was mapped using a Garmin GPS and Humminbird depth sounder (± 1 m) through ice holes and through the ice pan in 2003 and 2004 (Figure 3.1b). Based on the bathymetry, traps were deployed in the deepest known location in West Lake prior to runoff in 2003 (“Mid” site). After initial results from 2003, a second site (“Proximal” site) was established for 2004 and 2005.
At each site, sediment that settled out of the water column was
trapped to measure suspended sediment deposition (SSD) at frequent time intervals.
The sediment traps were constructed of a replaceable receptacle
mounted with a funnel with vertical walls to reduce turbulence along the upper edge of the funnel and minimize the potential for sediment to settle along the sides of the funnel (Figure 3.2). In 2003, the traps were changed daily during the peak period and then as infrequently as once a week afterwards. In 2004 and 2005 the traps were collected and redeployed daily during the peak discharge period and reduced to bi-daily intervals afterwards. The traps were moored at 51
0.5 m (lower trap) and 15 m (upper trap) from the lake bottom at each location in order to evaluate how sediment was distributed through the water column (Figure 3.2).
Hole through ice
Anchor across the hole to secure line
2 l polyethylene bottle with the bottom cut off 2 small holes in the plastic with a leader fed through to secure the line
Water Column 15 m from bottom
50 mL centrifuge tube Line Weight
0.5 m from bottom Lake Bottom
Figure 3.2: Schematic of the suspended sediment trap system deployed at Cape Bounty.
The trap receptacles were retrieved and separated from the funnels, sealed with headspace water and returned to the laboratory where they were filtered through pre-weighed 0.4 µm Isopore polycarbonate filters, oven dried (50ºC) and re-weighed to determine dry mass accumulation.
Particle size
analysis of the trapped sediment was carried out with the filtered sediment 52
samples after pretreatment with 30% hydrogen peroxide to digest organic material. Removal of the sediment from the filters was facilitated by the smooth surface provided by the laser-drilled polycarbonate filter material.
After
pretreatment, the samples were rinsed with distilled water into a Beckman Coulter LS200 laser particle size analyzer equipped with a fluid module. Each sample was analysed for 60 seconds with sonication, three times successively and the unaveraged third run was retained. Individual trap samples from the peak runoff period provided sufficient material for the particle size analyzer to calculate the grain size distribution for the lower trap at each site on a daily basis (>30 mg). However, reduced available sediment mass precluded daily particle size characterization of the 2004 upper traps and in both trap sets from 2005. In these cases, successive daily samples were combined until sufficient material was present to obtain reliable results with the particle size analyser. In addition to sediment trapping, water temperature (resolution 0.01ºC, ± 0.1ºC) was recorded at 20-minute intervals deployed 0.5 m above the sediment water interface at the Mid site with a Sequoia LISST-100 CTD. Progressive lens obscuration precluded use of the in-situ particle size information from the CTD. In 2005, two Hobo Water Temp Pro loggers (resolution 0.02ºC, ± 0.2ºC) were also deployed to monitor temperature in the water column at 1 m (not shown) and 15 m (mid-column) above the sediment water interface at the Mid site. To isolate short-lived fluctuations in lake bottom temperature from the seasonal
53
background warming trend, temperature departures (Td) were calculated as follows: Td = Tn -Tn-1
(1)
where Td was the calculated temperature departure, and Tn and Tn-1 were the sequential measured temperature values (ºC).
3.5 Results
The results reported in this study were collected as part of a comprehensive watershed monitoring program established at Cape Bounty in 2003.
Field
observations were carried out to obtain relevant data to support investigations of the linkages between meteorological, hydrological, and limnological processes that contribute to the sedimentary record. Detailed analysis of sediment delivery characteristics and hysteresis in the West River are described in McDonald (2007) and climatic controls over seasonal sediment yield in the West and adjacent East catchments (unofficial names) are assessed by Cockburn and Lamoureux 2008a.
3.5.1 Hydrometeorology
Snowcover and snow-water equivalence varied substantially between the three seasons.
2004 had the largest SWE estimates and a relatively continuous
snowcover in early June (Table 3.1). Snowcover was patchy and SWE was lower in 2003 and 2005 (Table 3.1). In 2003 and 2004, initial snowpack melt ponded in portions of the river channel prior to flow channelization due to 54
temporary snowpack dams. The ponded meltwater built up substantial runoff potential and subsequent runoff was intense (Lamoureux et al. 2006; Cockburn and Lamoureux, 2008a). The meltwater progressively incised into the snowpack and reached the channel bottom during or after the peak spring discharge. Initial melt and flow channelization was substantially different in 2005, due to warmer spring temperatures.
Channelization occurred within a 24-hour period and
temporary ponds did not form. Thermal conditions in 2005 were more favourable for rapid and continuous melt, as opposed to the cooler conditions in the previous years (Table 3.1) which led to episodic melt water generation and significant meltwater storage in temporary ponds.
As a result, runoff intensity was
substantially less intense in 2005 and flow remained on a snow-lined channel through the initial runoff period and isolated from potential sediment sources on the channel bed during peak runoff (Lamoureux et al. 2006). In addition to the warmer conditions (Table 3.1), the thin 2005 snowpack melted and fragmented rapidly, which further contributed to the moderate runoff intensity and reduced total runoff volume (McDonald 2007; Cockburn and Lamoureux 2008a). Table 3.1 Mean June temperature at Cape Bounty, snow-water equivalence (SWE), total discharge and suspended sediment yield for the West River during 2003-2005 at Cape Bounty. Total Mean June Suspended Estimate Total Runoff Year Temperature Sediment SWE (mm) (mm) (ºC) Yield (Mg) 2003 -0.9 43 69 134 2004 -0.1 82 120 413 2005 2.0 55 81 63 55
Runoff and sediment delivery to West Lake began in mid June in 2003 and 2004 and early June 2005. In 2003 and 2004, peak suspended sediment concentrations generally coincided with peak discharge and occurred 2-4 days afterwards (Figures 3.3, 3.4). In 2005 suspended sediment concentrations were comparatively low (Figure 3.5).
The larger lag between peak runoff and
suspended sediment concentrations observed in 2005 was due to flow that was isolated from the channel bed and potential sediment supplies for the majority of the runoff period (McDonald 2007; Cockburn and Lamoureux 2008a).
This
contrasts the observations from 2003 and 2004, where flow reached the channel bed and sediment supplies relatively quickly (McDonald 2007). Additionally, the magnitude of peak discharge and suspended sediment yield in 2005 was considerably less than in 2003 and 2004. This is attributed to the decreased runoff intensity due to continuous melt without ponding and a reduced and fragmented snowpack (Table 3.1; Cockburn and Lamoureux 2008a).
56
12 10 8 6 4 2 0 -2
1.0 0.5 0.0
2000
(d)
1500 1000 500 0
0.60 0.55 0.50 0.45 0.40 0.35 0.30
Instruments removed July 1, 2003
0.08
o
(f)
Temperature ( C)
Lake Bottom o Temperature ( C)
0.04 0.00
Instruments removed July 1, 2003
-0.04 -0.08
0.5 Upper Lake Trap 0.4 0.3
150
0.2
100
0.1
50
0.0 Suspended Sediment Discharge (Mg)
(h)
200
Mid
200
4
Lower Lake Trap
3
Mid
150
2
100 1
50 0 Jun 20 Jun 24 Jun 28 Jul 02
0 Jul 06
Jul 10
Date
Figure 3.3, see caption below, next page.
57
Jul 14
Jul 18
Jul 22
Jul 26
Jul 30
0
Suspended Sediment Discharge (Mg)
Cumulative . -2 Deposition (mg cm )
(g)
Cumulative . -2 Deposition (mg cm )
(e)
Suspended Sediment . -1 Concentration (mg L )
3. -2
(c)
Rainfall (mm)
1.5
(b)
0 5 10 15
Hourly River Temperature (oC)
Hourly Air o Temperature ( C)
15 10 5 0 -5 -10
Discharge (m s )
(a)
Figure 3.3: West Lake seasonal inflow and depositional summaries for 2003. (a) Hourly air temperature and daily rain fall, (b) hourly river temperature, (c) hourly river discharge, (d) hourly suspended sediment concentration in the river, (e) lake bottom temperature (f) lake bottom temperature departures, (g) cumulative suspended sediment deposition in the upper water column trap (20 m depth; bar graph), cumulative suspended sediment discharge from the river (solid line) and (h) cumulative suspended sediment deposition in the lower water column trap (33.5 m depth; bar graph) and cumulative suspended sediment discharge from the river (solid line).
58
1.2 0.8 0.0
(f)
1.2 1.0 0.8 0.6
Upper Lake Traps 500 Proximal Mid
400 300
0.4
200
0.2
100
0.0
Lower Lake Traps
(h)
Cumulative Suspended Sediment Discharge (Mg)
0.15 0.10 0.05 0.00 -0.05 -0.10 -0.15
500 400
14
0
12 10
Proximal Mid
8
300
6
200
4
100
2
0
0
Jun 20 Jun 24 Jun 28 Jul 02 Jul 06 Jul 10 Jul 14 Jul 18 Jul 22 Jul 26 Jul 30 Aug 03Aug 07
Date
Figure 3.4, see caption next page.
59
Cumulative Suspended Sediment Discharge (Mg)
Cumulative . -2 Deposition (mg cm )
(g)
5000 4000 3000 2000 1000 0
Suspended Sediment . -2 Concentration (mg L )
0.4
Temperature (oC)
3. -1
Discharge (m s )
1.2 1.0 0.8 0.6 0.4 0.2 0.0
(e)
12 10 8 6 4 2 0 -2
0 -4
1.6
(d)
Rainfall (mm)
4
0 5 10 15
Houlry River o Temperature ( C)
8
Lake Bottom o Temperature ( C)
(c)
12
Cumulative . -2 Deposition (mg cm )
(b)
Houlry Air o Temperature ( C)
(a)
Figure 3.4: West Lake seasonal inflow and depositional summaries for 2004. (a) Hourly air temperature and daily rain fall, (b) hourly river temperature, (c) hourly river discharge, (d) hourly suspended sediment concentration in the river, (e) lake bottom temperature (f) lake bottom temperature departures, (g) cumulative suspended sediment deposition in the upper water column trap (20 m depth; Proximal site dark gray bars, Mid site light gray bars) and cumulative suspended sediment discharge from the river (solid line) and (h) cumulative suspended sediment deposition in the lower water column trap (33.5 m depth; Proximal site dark bars, Mid site light bars) and cumulative suspended sediment discharge from the river (solid line).
60
8 6 4 2 0 -2
3 2 1
(d)
Lake Column o Temperature ( C)
(e)
(f)
0
1.0 0.8 0.6 0.4 0.2 0.0
0.9 0.8 0.7 0.6 0.5 0.4
500 400 300 200 100 0 Bottom temp Mid-column temp
0.06 0.04 0.02 0.00 -0.02 -0.04 -0.06
0.4
Upper Lake Trap 0.3
Proximal Mid
80 60
0.2
40 0.1
20 1.4
0
Lower Lake Trap
1.2
Proximal Mid
1.0
Cumulative . -2 Deposition (mg cm )
Cumulative . -2 Deposition (mg cm )
(g)
0.0 Cumulative Suspended Sediment Discharge (Mg)
(h)
80 60
0.8 0.6
40
0.4 20
0.2
0
0.0
Jun 05 Jun 07 Jun 09 Jun 11 Jun 13 Jun 15 Jun 17 Jun 19 Jun 21 Jun 23 Jun 25 Jun 27
Date
Figure 3.5, see caption next page.
61
Cumulative Suspended Sediment Discharge (Mg)
3 Discharge (m )
(c)
o Temperature ( C)
4
Hourly River o Temperature ( C)
No precipitation
Suspended Sediment Concentration (mg/L)
(b)
Hourly Air o Temperature ( C)
(a)
Figure 3.5: West Lake seasonal inflow and depositional summaries for 2005. (a) Hourly air temperature and daily rain fall, (b) hourly river temperature, (c) hourly river discharge, (d) hourly suspended sediment concentration in the river, (e) lake bottom temperature (f) lake bottom temperature departures, (g) cumulative suspended sediment deposition in the upper water column trap (20 m depth; Proximal site dark gray bars, Mid site light gray bars) and cumulative suspended sediment discharge from the river (solid line) and (h) cumulative suspended sediment deposition in the lower water column trap (33.5 m depth; Proximal site dark bars, Mid site light bars) and cumulative suspended sediment discharge from the river (solid line).
62
3.5.2 Sediment deposition rates and patterns
When channelized runoff initially reached the lake, the ice within 100 m of the delta flooded temporarily (1-2 days). The lake ice at the delta rapidly melted and the remainder of the lake-ice pan lifted from the shore, which allowed river flow to enter the lake unimpeded by the lake ice thereafter.
The ice pan persisted
through to at least mid-August (based on field observations and satellite imagery) each year. Suspended sediment deposition (SSD) from traps generally corresponded with suspended sediment concentrations (SSC) in the river and the resultant cumulative river suspended sediment discharge curve mirrored the cumulative SSD profile in all three years (Figures 3.3-3.5). The periods of highest SSD were associated with periods of the highest SSC in the river. In 2003 and 2004, these periods of high sediment inflow were associated with temperature perturbations up to 0.12ºC in the lake bottom (Figures 3.3, 3.4). By comparison, the bottom temperature departures were less frequent in 2005 and were substantially lower magnitude with a maximum absolute value 0.05ºC. Temperature in 2005 was stable to the resolution of the instrument (0.01ºC) for several periods of 24 hours or more. The multi-level trap deployment generated daily and near-daily records of suspended sediment deposition in the upper and lower water columns (Figures 3.3-3.5 bottom two panels (g, h)). In general, deposition was greatest in the 63
lower trap compared to the upper water column trap as to be expected based on the relative depths of the overlying water columns, with the lower traps exposed to sedimentation from approximately two times the potential suspension deposition. The lower Proximal monitoring site received the most sediment each year and the upper Mid monitoring site received the least sediment each year (Figures 3.3-3.5; Table 3.2). However, total deposition in the lower trap was not twice the deposition in the upper trap for a given season, which indicated that sediment was not distributed uniformly through the water column. Furthermore, the ratio of lower to upper deposition varied substantially, and included instances where deposition patterns were inverted (ratio
