Suspended sediment transport effectiveness of three ... - Springer Link

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Suspended sediment transport effectiveness of three large impounded rivers, U.S. Gulf Coastal Plain Paul F. Hudson 7 Joann Mossa

Abstract Suspended sediment transport effectiveness was examined near the mouths of three large impounded rivers (Rio Grande, Brazos, and Pearl Rivers) in differing precipitation regimes in the U.S. Gulf Coastal Plain. Magnitude and frequency analysis of suspended sediment transport was performed by examining the effectiveness of both discharge and time in transporting suspended sediment. Bivariate plots of discharge with infrequent values of silt/clay and sand provide an insight into the relative proportion of coarse-versus finegrained sediment transport for the three rivers. Despite the aridity of the Rio Grande and Brazos drainage basins, which is often associated with infrequent or episodic transport of sediment, the duration of the effective discharge is similar to humid basins described in the literature. The majority of sediment transport occurs during the moderate discharge events, having a duration of 2.4%, 1.5%, and 4.4% for the Rio Grande, Brazos, and Pearl Rivers, respectively. This may be due to the influence of scale or the influence of upstream dams and reservoirs on discharge and sediment transport. Findings from this research suggest that magnitude and frequency analysis of discharge and suspended sediment near the mouths of large rivers may provide a useful framework for understanding the timing and delivery of riverine sediments to the nearshore coastal environment from rivers draining a range of geologic and climatic settings. Key words Suspended sediment transport 7 Impoundments 7 Magnitude-frequency analysis 7 Effective discharge

Received: 6 September 1996 7 Accepted: 4 February 1997 P. F. Hudson (Y) Department of Geography and Anthropology, Louisiana State University, Baton Rouge, LA, 70803-4105 USA J. Mossa Department of Geography, University of Florida, Gainesville, FL, USA

Introduction Rivers are considered major sources of nearshore sediments (Carter 1988) which may dominate the coastal environment near the mouths of large rivers (Wiseman and Garvine 1995). Having an understanding of the timing and delivery of sediment transported to the coastal setting is increasingly recognized as being important for a variety of concerns. Estuarine ecosystems are sustained due to a continual supply of riverine sediments (Hutchinson and others 1995) which may be transported along the coast by a variety of processes upon being dispersed into the marine setting (Wright and Nittouer 1995). In addition to their ecological significance, the transfer of fluvial sediment-associated pollutants into the nearshore zone is also of concern (Chambers and others 1995; Collins and others 1995; Patrick 1994). With greater pressure put on the coastal zone it remains essential to establish linkages between the supply of riverine sediments and the environments they impact. Thus an understanding of the significance of specific discharge classes in transporting sediment above the mouths of large rivers flowing into the northern Gulf of Mexico may provide insight into the delivery of terreginous clastic sediments supplied to the coastal zone. Wolman and Miller (1960) used a unique approach to investigate the effectiveness of streams in transporting sediment and whether events of high magnitude and low frequency, or events having a moderate magnitude but at a higher frequency of occurrence, are more responsible for the majority of sediment transport in rivers. The effective discharge was defined as the discharge, or class of discharge, which transports the most sediment. Their research concluded that a stream tends to transport most of its suspended sediment load over a range of discharges, with the most effective discharge being of moderate magnitude and having a fairly frequent rate of occurrence, usually once to twice per year. More recent studies of undisturbed rivers have concluded that the effective discharge is highly variable, ranging from days to decades, and attributed to the immense diversity of discharge, sediment, channel, and basin characteristics of fluvial systems (Nash 1994). Since Wolman and Miller (1960) there have been relatively few studies which have evaluated this issue for large river basins, particularly those with dams and reservoirs. This study examines the effectiveness of discharge classes in transporting susEnvironmental Geology 32 (4) November 1997 7 Q Springer-Verlag

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Fig. 1 Hydrography showing drainage for the western Gulf Coastal Plain and gauging stations for the Rio Grande, Brazos, and Pearl Rivers (Data source is 1 : 2 000 000 scale USGS digital line graphs)

pended sediment for three large impounded rivers draining into the northern Gulf of Mexico, namely the Rio Grande, Brazos, and Pearl Rivers (Fig. 1). The objectives of this study are: (1) to determine the time and discharge conditions for transporting 10%, 25%, 50%, 75%, and 90% of the total suspended sediment load; and (2) to determine the most effective discharge class and its duration for transporting suspended sediment. Such work can provide a better understanding of the behavior of large impounded rivers and will provide some insight into the delivery of sediments to the nearshore zone of the northern Gulf of Mexico.

Wolman and Miller (1960) has been sparse (Nash 1994). Much of this is probably due to the lack of long-term data bases with abundant measurements which span the entire discharge range (Webb and Walling 1982). Without numerous measurements, the potential for error is great because sediment transport is often estimated using regression relationships and there are not sufficient values to include in each discharge class. Prior to the mid-1980s most research on this topic was done in humid settings, and consequently tended to support the findings of Wolman and Miller (1960). However, Wolman and Miller (1960) and others since have noted that more work may be performed by infrequent large events in arid areas having highly variable discharges. More recently, several studies have taken advantage of Previous research longer and more continuous sediment records, and have published findings which deviate from Wolman and MilIn the 37 years since Wolman and Miller’s (1960) classic ler (1960), suggesting that it is not possible to generalize study, research into the amount of work performed by about the return period for the effective discharge. For geomorphic events may be divided into two areas: (1) example, Webb and Walling (1982), researching small studies using a similar approach to Wolman and Miller, upland basins in Britain, found that 50% and 90% of susassessing the manner in which a stream transports its se- pended sediment load was transported in 0.8% and 6.0% diment load; and (2) studies which examine changes in of the time, respectively. Nolan and others (1987) examthe landscape due to geomorphic events. In the latter, re- ined several rivers in northwestern California and found currence intervals have been cited for the effective disthat the recurrence interval of the most effective discharge which exceed those of Wolman and Miller (Beaty charge and 90% of total suspended sediment load was re1974; Schick 1974; Pickup and Warner 1976; Baker 1977; ported at 16.1 years, much greater than recurrence interWolman and Gerson 1978; Kochel 1988; Miller 1990). vals which had previously been reported. Ashmore and This is to be expected, because as a landform increases in Day (1988) examined the effective discharge ranges for 21 size or scale, from bedforms to cross-sectional form to stations in the Saskatchewan Basin. The duration of the planform to profile form, it has increased ability to abeffective discharge increased as drainage area increased sorb changes and requires longer timescales for adjustand flow variability decreased. Histograms of sediment ment (Knighton 1984; Schumm 1991). transported over chosen discharge ranges had a variety of Research concerning the magnitude and frequency of sus- forms, including bimodal, and complex forms which difpended sediment transport using methods similar to fered from the unimodal forms identified by Wolman 264

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Research article

and Miller (1960). Recently Nash (1994) provided a comprehensive study of the recurrence intervals of the effective discharge class for 55 undisturbed streams ranging in scale and climate in the U.S.A. Recurrence intervals in various regional settings of the most effective discharge class ranged from weeks to decades. This high variability suggests that the view that effective fluvial discharge occurs about once a year is misleading, and that it is not appropriate to propose a universal or widely applicable recurrence interval for effective discharge. Several recent studies have described how the number of river regulation and water development projects such as impoundments have escalated tremendously in the latter half of the twentieth century (Beaumont 1978; Walling 1987; Petts 1994). Although one of the major effects of impoundments is sediment trapping, some materials remain in suspension and new materials are introduced from downstream tributaries and portions of the main channel. Sediment transport downstream of impoundments has often been ignored, with many studies intentionally avoiding assessing the effectiveness of streams in transporting sediment that have been artificially disturbed (Nash 1994). Yet given the numerous positive and negative effects of sediments, knowledge of the effectiveness of sediment transport downstream of impoundments is no less important than that upstream of impoundments or in streams without impoundments. As human modifications of rivers continue, more studies are needed in modified systems, including those with numerous impoundments. Given that impoundments are constructed for a wide variety of purposes, including flood control, water supply, hydropower, and recreation, such studies should examine rivers in a wide range of climatic and geologic conditions.

Physical setting Climate The Rio Grande, Brazos, and Pearl River basins are located within a similar latitude, thus mean temperatures vary little for the three basins. However, precipitation for

the three basins varies markedly, becoming increasingly humid in a west to east direction (Fig. 2). Precipitation within the drainage basins varies as well. The Brazos and Rio Grande display more variability in precipitation than the Pearl River, becoming increasingly humid as they flow east towards the Gulf of Mexico. The Pearl River, located in central Mississippi and southern Louisiana has uniformly high precipitation. Geology The Rio Grande drains over 453 250 km 2 and includes parts of Colorado, New Mexico, Texas, and four Mexican states (Table 1). Surficial geology of the upper and middle basin is varied, with rocks of Precambrian to Quaternary age and of sedimentary, igneous, and metamorphic origin. The lower basin is composed of predominantly Cretaceous deposits. The bed-material size decreases in a downstream direction from a sand-gravel channel to a channel comprised of sandy silt (Culbertson and Dawdy 1964; Belcher 1975). The Brazos River drains an area of 113 960 km 2, although 23 310 km 2 is considered noncontributing. The upper reaches of the Brazos are comprised of the easily erodible Permian red beds, causing the channel to assume a braided pattern. In the middle portion of the basin, the channel is incised in Pennsylvanian and Cretaceous limestone. As the river flows through the Gulf Coastal Plain the river assumes a meandering pattern (Strickland 1961; Wooly 1985). Table 1 Drainage basin and data characteristics Variable

Rio Grande at Brownsville, TX 1966–1983

Brazos at Richmond, TX 19666–1986

Pearl at Bogalusa, LA 1967–1989

Drainage area (km 2) Nr. of Days Mean Q (m 3/s) Mean SSQ (tonnes/day) C.V. (std./mean) Runoff (cm/year)

453 250.0 6 330.0 47.0 1 913.1 1.8 0.3

113 960.0 7 554.0 193.6 27 515.1 1.4 5.4

22 455.0 7 013.0 311.4 3 597.2 1.2 47.7

Fig. 2 Annual precipitation (cm) in the Gulf Coast region. (Data source: Climates of the States, 1974)


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The Pearl River drains 22 455 km 2 of Mississippi and southeastern Louisiana (Wiche and others 1988). The geology of the basin is predominantly Quaternary and Tertiary clastic sediments. The valley bottom shows multiple interconnecting channels separated by densely vegetated islands, or an anastomosing river pattern. Many of the channels are stagnant at low water, but experience flow during periods of high stage. Dams and reservoirs are located on the main channel of all three rivers. Through 1978, in Texas alone, 10 impoundments have been constructed on the Rio Grande and its tributaries since 1911, and 33 have been constructed on the Brazos River and its tributaries since 1921 (Texas Water Development Board 1974; Hudson 1993; Mossa and others 1993). Several others, some of which have been described by Williams and Wolman (1984) occur in New Mexico, Colorado, and Mexican states. Only one impoundment, the Ross Barnett Dam and Reservoir, occurs on the Pearl River and was put into operation in 1961 near Jackson, Mississippi (Wiche and others 1988). The three stations analyzed in this study were chosen due to (1) the length of data record for daily suspended sediment, which is essential to capture the variability in a system, particularly for arid rivers; and (2) the stations being located near the mouths of large rivers, providing a more accurate characterization of the dynamics between discharge and suspended sediment just before entering the coastal zone. Various studies have described how impoundments affect the discharge and sediment transport of rivers. Effects on the discharge regime typically include a reduction in the number of high and low discharges and an increase in the number of moderate discharges. However, the type of change depends upon the usage of the dam and reservoir (Williams and Wolman 1984). Typical uses of such structures for the three rivers in the study include irrigation, hydroelectric power generation, flood control, downstream sediment reduction, public water supply, and recreation (Wells and others 1988; Carr and others 1990; Ong and others 1991). Dams used for irrigation may generate short-term variable flows during peak demand, and relatively moderate flows the remainder of the time. Hydroelectric dams may cause widely fluctuating flows (Williams and Wolman 1984).

urements of suspended sediment concentrations in a cross section. U. S. Geological Survey sampling methods incorporate both depth-integrated and point-integrated techniques, which account for cross-sectional variations in suspended sediment concentration. Compared to water discharge, measurements of suspended sediment used to estimate daily values are lacking throughout much of the Gulf region. Although several locations on numerous rivers are sampled infrequently, other than the Lower Mississippi-Atchafalaya systems only three large rivers draining into the northern Gulf of Mexico have gauging stations near the river mouth with daily or frequent measurements for a substantial number of years. Periods of missing data, although not significant, existed for all three stations. Missing data ranged from a few months for the Rio Grande at Brownsville, to a little over a year for the Pearl River at Bogalusa. Box-plots were used to graphically illustrate differences in duration for discharge and suspended sediment transport for the three rivers. Magnitude-frequency analysis was performed using techniques similar to Wolman and Miller (1960). Rather than using even class intervals as has been generally reported in the literature (Benson and Thomas 1966; Neff 1967; Andrews 1980; Webb and Walling 1982; Nolan and others 1987; Ashmore and Day 1988), class ranges suggested by Searcy (1959) were utilized. These class ranges adjust for the skewed nature of hydrologic data by grouping the high number of low discharges into smaller classes. Once these classes were chosen, the amount of sediment transported by ranges of discharge was determined, where the effective discharge plots as the peak on the sediment discharge histogram.

Results

Discharge variability, shown by the coefficient of variation (Table 1) and the box-plot of time required to transport flows (Fig. 3), increases in a westerly direction with decreasing precipitation. For example, the Rio Grande, the basin with the lowest precipitation, required 32.7% of the time to transport 90% of its discharge. In contrast, the Pearl River, the station with the highest amount of precipitation, required 56.5% of the total time to transport 90% of its discharge (Fig. 3). This is not unexpected, and is in agreement with Baker (1977), who found hydroData and methodology graphs to become increasingly skewed in a westerly direction from Louisiana to western Texas for rivers rangOnly discharge and suspended sediment data collected by ing in size from 1300 to 2500 km 2. the U.S. Geological Survey are utilized in this study. Wa- Discharge variability appears to affect the transport of ter quality data collected by state agencies may not be re- suspended sediment, as observed in the sediment duraliable because the samplers that are used are generally tion curves for the three stations (Fig. 4). The lower end improperly designed to accommodate the kinematics of of the tails on the right side of the graphs becomes steepflowing water. Because most suspended sand travels near er for rivers with decreasing precipitation. This indicates the channel bed, as compared to silt and clay which are that in arid basins less of the total sediment is transwell-mixed throughout a river cross section (Colby 1963; ported by the more frequent lower discharges, and that Nordin and Dempster 1963), it is important to include less frequent higher flows are responsible for transporting samples close to the channel bed to obtain reliable meas- a larger portion of the sediment. Thus the amount of

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Fig. 3 Box-plots showing the percent of time required for each river to transport 10%, 25%, 50%, 75%, and 90% of its discharge

Fig. 5 Box-plots showing the percent of time required for each river to transport 10%, 25%, 50%, 75%, and 90% of its total sediment load

Fig. 4 Discharge (m 3/s) and suspended sediment (tonnes/day) duration curves showing the percentage of time a specific discharge or sediment load is equaled or exceeded

Fig. 6 Cumulative discharge and suspended sediment duration curves. Steeper segments of curves indicate that a greater portion of the total discharge or sediment load is transported over a short period of time

time required to transport percentages (10%, 25%, 50%, 75%, and 90%) of the total sediment load decreases in a westerly direction (Fig. 5). Cumulative sediment duration curves (Fig. 6) show the portion of the total sediment load transported during the cumulative percentage of time. Steeper curves indicate that a greater portion of sediment for a river is transported over a shorter period of time. For example, the Rio Grande transported 90% of its sediment load in 17% of cumulative time, the Brazos transported the same amount in 21%, while the Pearl required almost twice as much time as the Rio Grande, transporting 90% of its sediment load in 33% of the cumulative time.

Although a spatial climatic pattern related to precipitation and discharge variability was observed between suspended sediment and time, suspended sediment and discharge did not display the same trend (Fig. 7). The percent of discharge required to transport 50% and 90% of the suspended sediment load for the three rivers was 38% and 75%, 24% and 64%, and 43% and 77% for the Rio Grande, Brazos, and Pearl Rivers, respectively. The percentage of discharge required to transport suspended sediment for the Brazos is lower than that of the Rio Grande. One possible explanation is the role of scale, as the drainage area of the Brazos is smaller than the Rio Grande, allowing for more sediment storage along the Environmental Geology 32 (4) November 1997 7 Q Springer-Verlag

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Fig. 7 Box-plots showing the percent of discharge required for each river to transport 10%, 25%, 50%, 75%, and 90% of its total sediment load

Rio Grande. Another explanation is climate, because sediment yields are generally greater in semi-arid basins than arid and humid basins (Langbein and Schumm 1958), and the highest discharges on the Brazos have higher suspended sediment concentrations than on the Pearl or Rio Grande (Mossa and others 1993). Alternatively, impoundments provide an additional explanation because the station on the Brazos is further downstream of main channel impoundments than the other rivers so that sediment production might be more efficient during high discharges. For all three rivers the percentage of discharge required to transport 50% and 90% of the sediment is much higher than that reported by Webb and Walling (1982) for a small basin in Britain, which likely indicates the influence of scale on relationships between discharge and suspended sediment. The shape of the sediment discharge histograms vary for the three rivers (Fig. 8), showing differences in the timing and distribution of sediment transported over the range of discharge classes. As would be expected in basins as arid as the Rio Grande and Brazos (Wolman and Miller 1960), lower discharges are less effective in transporting sediment, causing the effective discharge to be shifted toward higher discharge classes (Baker 1977). In contrast, sediment transport in the Pearl River occurs over a range of discharge classes, with low discharges also contributing to sediment transport. The most effective discharge class for the Rio Grande, Brazos, and Pearl Rivers occurs at the second, third, and fourth highest discharge class, respectively.

Discussion In spite of differences in precipitation, drainage area, and impoundments for the three rivers, the majority of sediment is transported by relatively frequent discharge 268


Fig. 8 Sediment discharge histograms for the a Rio Grande at Brownsville, TX, 1966–1983; b Brazos River at Richmond, TX, 1966–1986; and c Pearl River near Bogalusa, LA, 1967–1989. The graphs show the frequency of discharge events within each class and the amount of suspended sediment transported by classes of discharge. The lower limit of the class interval is given on the x-axis. The total suspended sediment discharge (SSQ) in tonnes (t) per discharge class is computed by summing the daily sediment load values which occurred between the lower and upper discharge values for each discharge class. The effective discharge plots as the peak in total SSQ on the line graph

Research article

events. The duration of the effective discharge for the Rio Grande, Brazos, and Pearl Rivers was 2.4%, 1.5%, and 4.4%, respectively. Although the size of the drainage basins in this study are greater than most found in the literature, the duration of the effective discharge appears to be comparable. However it is somewhat difficult to compare these findings due to the variety of ways in which the effective discharge has been reported (Nolan and others 1987; Ashmore and Day 1988; Nash 1994). Several studies have reported figures for the class of discharge which transports the most sediment (the effective discharge) (Benson and Thomas 1966; Andrews 1980; Nolan and other 1987; Ashmore and Day 1988; Nash 1994), while others have reported recurrence intervals for percentages of suspended sediment transported (Wolman and Miller 1960; Webb and Walling 1982; Nolan and others 1987). The approach used by this study may be more appropriate for several of reasons. Box-plots provide an improvement over conventional tables by providing a direct comparison of durations for discharge and suspended sediment for different rivers. Secondly, considering the skewed nature of hydrologic data, Searcy’s (1959) method groups the high number of low discharge events into smaller classes. This adjustment may cause the effective discharge class (the modal class) to shift towards the higher discharge ranges, and implies that the duration of the effective discharge reported by previous studies may decrease due to being associated with higher discharges. The discussion thus far has primarily considered sediment transport with respect to discharge variability. However, differences in the shape of the sediment discharge histograms (Fig. 8) should also be considered with respect to differences in particle entrainment and suspension between coarse- and fine-grained sediments, as well as differences in the sediment regime between arid and humid settings. In contrast to rivers draining humid settings, rivers in more arid settings tend to transport a greater proportion of their total suspended sediment load as coarse-grained sediments. However, particle size is also of importance, as the manner in which fine and coarse sediments are transported varies considerably (Bagnold 1966). The wash load component of the suspended sediment load is not as strongly related to stream power as are the larger particles entrained from the bed of the channel (Colby 1963). There are several reasons for this. While sediments larger than 0.0625 mm are scoured from the channel bed, the wash load (silt/clay) is continuously suspended within the water column and is generally supplied from upstream sources. Secondly, stream power and sediment entrainment are inversely related for loose particles with a diameter smaller than medium sand (0.25 mm) , therefore requiring increasing amounts of velocity for entrainment (Colby 1963; Nordin and Dempster 1963). While daily records of particle size are not available, analysis of bivariate plots of discharge versus infrequent measurements of silt/clay and sand for the three rivers adds insight into the influence of the sediment regime on the shape of the sediment discharge histograms (Fig. 9).

Bivariate plots between discharge and suspended sediment reveal that the Rio Grande near Brownsville has a concave upward pattern (Fig. 9a). The pattern persists for both the silt/clay and sand components of the sediment load. This illustrates that lower discharges are ineffective at entraining bed material, and also transport a relatively small proportion of the fine sediment load, possibly due to flocculation of clays causing aggregate particles to behave as larger sediments. As discharge increases, a threshold is reached where sediment is entrained and transported through the water column. The higher amount of scatter associated with the sand may be due to turbulence, or, a greater range of particle sizes which are less easily continuously suspended throughout the water column, as is the wash load. Compared to the Rio Grande near Brownsville, the Brazos River at Richmond exhibits a stronger relationship with discharge for both silt/clay and sand (Fig. 9b). However, unlike the Rio Grande, at approximately 56 m 3/s (2000 ft 3/s) a threshold is reached where the rate of sand concentration increase slightly declines with higher discharges. This may help to explain the difference between the sediment discharge histograms for the two rivers. The most effective discharge class for the Rio Grande and Brazos Rivers are the second and third highest peaks, respectively. Although, the decreasing scatter associated with higher discharges indicates that the channel is efficient at entraining bed material. The silt/clay component exhibits a slight concave downward pattern for discharges greater then 1000 m 3/s, possibly indicating that this component of the suspended sediment load has passed through the system prior to the peak of large flood events. Finally, analysis of the silt/clay and sand components of the sediment load for the Pearl River at Bogalusa reveals a much different trend (Fig. 9c). The silt/clay component shows a fairly linear relationship with discharge. However, the sand component has a concave downward pattern, which indicates that exhaustion and dilution of the sand component of the suspended sediment load is occurring during large peak flood events. In spite of the direct impact that impoundments can be said to have on the discharge and sediment regime of a river, it may be difficult to establish the manner in which these changes may be manifest on sediment transport effectiveness. Williams and Wolman (1984) mention that dams along the main channel of the Rio Grande have been built specifically to reduce downstream sedimentation, and cite trap efficiencies up to 99% for reservoirs located along other large rivers in semi-arid climates. However, the effects of sediment trapping tend to decrease downstream, and sediment loads may recover to pre-dam conditions (Richards 1982). The distance needed for sediment loads to recover downstream may vary based on the input of tributaries and the size of sediment in the system. For large rivers it may take up to 500 km before the sediment load recovers to pre-dam levels, and in some cases sediment loads may not recover (Williams and Wolman 1984). Sediment measurement stations for the Rio Grande, Brazos, and Pearl Rivers are approxiEnvironmental Geology 32 (4) November 1997 7 Q Springer-Verlag

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Fig. 9 Bivariate plots of discharge and suspended sediment concentration of silt/clay and sand for the a Rio Grande at Brownsville, TX, 1967–1992; b Brazos River at Richmond, TX, 1969–1992; and c Pearl River near Bogalusa, LA, 1984–1991 (STCLCON silt/clay concentration, SANDCON sand concentration)

mately 325, 515, and 370 km downstream of main stem dams, respectively. Characteristics of impoundments that are likely to be important include modes of operation, spatial distribution, and distance between the large main stem impoundments and the sediment measurement station. There are several ways in which the duration of the effective discharge may be impacted by impoundments. The first is that there is a reduction in the number of high discharge events, increasing the number of moderate discharge events. Secondly, the reduction in high discharge events downstream of dams may mean that entrainment 270


velocities will not be sufficient to flush large-sized sediments through the channel system (Knighton 1984). Finally, trapping of larger-sized sediments within reservoirs causes this component of the sediment load to be selectively removed from the system downstream of the dam. Combined, these factors indicate that during high discharge events there will be a reduction in both the supply of the coarse faction of the sediment load and in the peak discharge, in comparison to pre-dam levels. Thus, the role of impoundments in influencing the transport effectiveness of rivers is to reduce the duration of the effective discharge.

Research article

The suspended sediment loads for the rivers analyzed in this study is likely to decrease towards the coast due to storage in channel bottoms and estuarine wetlands (Phillips 1991). However, it remains necessary to consider how the sediment load may vary with discharge. Upon entering the marine setting there are two ways in which the discharge event may influence the distance that riverine sediment is transported before initial deposition. The buoyancy of the sediment plume is one of the primary determinants in how far sediments may be transported from the river mouth (Wright and others 1990). Plume buoyancy is determined by density differences between the plume and seawater, and is inversely related to the suspended sediment concentration of the plume (Wright and Nittrouer 1995). Rivers transporting a greater portion of their suspended sediment load during high discharge events, therefore, may have a sediment plume of a lower buoyancy (higher density relative to seawater), causing a greater proportion of the sediment to be deposited adjacent to the river mouth (Wright and others 1990). Secondly, the strength of large discharge events may cause the sediment transported into the marine environment to travel further before deceleration and deposition (Wiseman and Garvine 1995). Thus analysis of discharge and sediment dynamics employing the magnitude and frequency approach for stations near the mouths of large rivers may offer insight into the timing and delivery of sediments to the nearshore coastal environment. The integration of sediment-discharge histograms with plots of sand and silt/clay concentration provides a framework for evaluating the relative significance of coarse and fine sediment supply from floods as contrasted with moderate and low flows of a more frequent duration. Sediments may be supplied to a receiving basin from a variety of climatic settings. Therefore, this approach may aid in characterizing the individual magnitude-frequency signatures for large impounded rivers from arid to humid settings. Such information provides a context which is useful in evaluating the relative contribution of sediment supply from either locally derived sources, such as entrainment of bed material and channel scour, or from upstream sediment sources. This is of crucial importance with respect to water quality issues, particularly with regards to the transport of pollutants which are often adsorbed to fluvial sediments of specific sizes. The findings of this study are also of regional importance, in that they establish the effectiveness of discharge events in transporting sediment for large impounded rivers flowing into the northern Gulf of Mexico. Of the rivers flowing into the northern Gulf of Mexico, 24 have drainage basins over 2600 km 2 (1000 mi 2) in area. Of these 24 rivers, 19 have their flow regulated by dams and reservoirs located within their drainage basins. The majority of the literature on this topic has examined this issue for smaller rivers not heavily impacted by humans, usually located in the upper reaches of drainage basins (Webb and Walling 1982; Nolan and others 1987; Nash 1994). As this is not characteristic of the rivers discharging into the northern Gulf of Mexico, the current state of

knowledge regarding this topic may be inadequate when extended to issues related to the transport of sediments into the coastal environment near the mouths of large rivers. A prime example is the seafood industry of Texas, valued at over 1.25 billion U.S. dollars (Moody and others 1985). Section 11.47(a) of the Texas Water Code requires that inflows of freshwater be provided to bays and estuaries in order to preserve the sensitive ecosystems associated with these settings. Although it is recognized that riverine sediments are an essential component of these environments, the manner in which sediments contributing to these settings are related to river discharge remains poorly understood (Holley 1992).

Conclusions Magnitude and frequency analysis has been performed for three large impounded rivers flowing into the northern Gulf of Mexico in differing climatic regimes. The combination of sediment discharge histograms and bivariate plots of discharge and sediment provides insight into how the quantity and size of sediments transported may vary between small and large discharge events. Although box-plots and sediment discharge histograms reveal differences between the three rivers in the effectiveness of small and large discharges in transporting sediment, the patterns are similar to humid rivers frequently reported in the literature. Although the Rio Grande and Brazos Rivers exhibit the “potential” for catastrophic response, or, performing the most “work” during extreme events because of their aridity, the role of scale or possibly the influence of impoundments on the discharge and sediment regime likely increases the duration of the effective discharge. Thus, discharges having a moderate frequency of occurrence appear responsible for transporting most of the suspended sediment load in large impounded rivers in both arid and humid basins. Where adequate data is available, future research could employ the approach used in this study as a framework with which to assess separate magnitude-frequency relationships for different size classes of sediments. In rivers where data spans preand post-dam periods, studies should assess the downstream impact of impoundments on this topic. Such information may be useful with respect to engineering and management practices, so that environments which are effected by riverine sediments are not adversely impacted by modifications to the discharge and sediment regime of rivers caused by impoundments.

References Andrews ED (1980) Effective and bankfull discharges of streams in the Yampa River Basin, Colorado and Wyoming. J Hydrol 46 : 311–330


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