Journal of Hydrology 265 (2002) 149–163 www.elsevier.com/locate/jhydrol
Geochemical variations during flash flooding, Meramec River basin, May 2000 W.E. Winston*, R.E. Criss Department of Earth and Planetary Sciences, Washington University, St Louis, MO 63130, USA Received 1 October 2001; revised 22 April 2002; accepted 3 May 2002
Abstract Severe flooding in the Meramec River basin followed an extraordinary rainfall event on May 7, 2000. Precipitation measurements for the 13-h event ranged from 12.7 to 39.9 cm over a 4100 km2 region centered near Union, Missouri. Sample collections for isotopic and chemical analyses and field measurements of water temperature, specific conductivity, turbidity, and pH were made from three rivers during the course of the event. Relative to pre-storm values, flood water underwent a three to 10-fold decrease in conductivity, a 100-fold or more increase in turbidity, and pH fluctuations over a range of 1.0 unit. Concentration of major ions varied inversely (Ca, Mg, Na, Cl, SO4) or directly (K) proportional to discharge. Oxygen isotope measurements were used to separate each discharge hydrograph into pre-event and event water components. Event water dominated during peak runoff on each of the rivers, a condition atypical of floodwaters in this region and the result of overland flow or rapid subsurface delivery of precipitation. The Bourbeuse River showed the largest event water component during this exceptional event, with storm water making up essentially 100% of the flow for more than 24 h. q 2002 Elsevier Science B.V. All rights reserved. Keywords: Flash flood; Oxygen isotopes; Hydrograph separation; Water chemistry
1. Introduction Floods can cause widespread personal and economic loss, and major efforts are undertaken to reduce or relieve their effects. Seasonal precipitation cycles and snowmelt cause predictable, periodic flooding on rivers throughout the world, while intense localized rainfall can result in unexpected flash flooding during any season (Costa, 1987). Extensive flood control projects such as levees, dams and reservoirs have been constructed to contain the channels of major rivers * Corresponding author. Tel.: þ1-314-935-7475; fax: þ 1-314935-7361. E-mail address:
[email protected] (W.E. Winston).
and to impound excess water for later release. Even so, flooding in the United States still accounts for , 65% of all federally declared disasters and flash flooding is the major cause of weather related deaths (Smith and Ward, 1998). In fact, engineering efforts on extensively managed rivers have actually resulted in increased stage levels for the same discharge (Belt, 1975; Criss and Shock, 2001). River stage is the most common and widely reported parameter quantified during a major flood, but numerous less obvious hydrologic responses, such as changes in water chemistry, are also associated with flooding (Toler, 1965; Johnson et al., 1969; Walling and Foster, 1975; Caissie et al., 1996; Criss et al., 2001). Rising flood waters typically exhibit
0022-1694/02/$ - see front matter q 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 1 6 9 4 ( 0 2 ) 0 0 1 0 5 - 1
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Fig. 1. Map of east central Missouri indicating basin boundary (thick black line), sampling sites (open circles), and river gauging stations (open triangles). Inset shows the basin location in Missouri and the white box corresponds to map extent. Isohyets (hatched lines) are based on precipitation totals (cm) reported for each listed location. Distribution of the most extreme rainfall over the Bourbeuse River basin led to extensive damage in the town of Union, two fatalities nearby, and dramatic contributions of event water to river discharge.
increases in sediment load and negative, or sometimes positive, variations in the concentrations of some solutes. These chemical changes are due to the mixing of relatively dilute precipitation (event water) with pre-existing soil water and groundwater in the basin (Criss, 2002). Techniques have been developed to identify the relative contributions of ‘event’ and ‘preevent’ (baseflow) waters by using conservative natural isotopic or chemical tracers during the course of an event (Pinder and Jones, 1969; Sklash and Farvolden, 1979). Hydrograph separation using stable oxygen isotopes provide reliable results in many environments (Sklash and Farvolden, 1979; Pearce et al., 1986; Brown et al., 1999), but these studies have primarily focused on small headwater catchments and moderate storm events. The Meramec River basin in east central Missouri
(Fig. 1, inset) represents a unique opportunity to study the natural storm response of a significant river system (10,300 km2). In a historic public referendum in 1978, a major congressionally approved plan to construct five large reservoirs in the basin was ‘deauthorized’ so the river was spared the engineering works and flood management practices found on practically all other waterways in the United States (Jackson, 1984; Ruddy, 1992). Periodic sampling of surface and spring waters in the Meramec basin has been carried out by our research group since 1995. Long term isotopic analysis has resulted in an understanding of basin background conditions and identified the range of seasonal behavior against which individual flood events can be compared (Criss, 1999; Frederickson and Criss, 1999; Winston, 2002). Severe flash flooding occurred in east central
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Fig. 2. Discharge hydrographs for major rivers of the Meramec basin and hourly precipitation record for Union, Missouri. The storm was remarkably intense, delivering up to 40 cm of precipitation in only 13 h. Note the symmetric shape of the hydrographs including steep rising and recession limbs indicative of flash flooding. Data from USGS.
Missouri in response to an intense rainfall event on May 7, 2000 that delivered from 12.7 to 39.9 cm of rain over much of the lower Meramec River basin (Winston and Criss, 2002b). This event provides an opportunity to apply principles of isotopic hydrograph separation to an exceptional hydrologic event in a large watershed and to examine changes in the chemistry of floodwaters. Hydrograph separation using oxygen isotopes are performed on floodwater in the Meramec, Bourbeuse and Big rivers, and are then related to fluctuations in specific conductivity, pH, turbidity, and the concentration of major ions (Ca, Mg, Na, K, Cl, SO4) over the course of the event. 1.1. Event synopsis A detailed account of the extraordinary May 7 storm and its hydrologic response has been published elsewhere (Winston and Criss, 2002b) and is briefly summarized here. Precipitation began gradually at Union on the evening of May 6 and became more intense after midnight when rainfall rate exceeded 2.5 cm h21 until 7:00 am. The most intense precipitation was recorded between 2:00 and 3:00 am when over 7.6 cm fell. Rainfall totals across the basin ranged from 0.5 cm at Steelville to 39.9 cm in the city of Union (Fig. 1). Infiltration capacity in the lower basin was rapidly overwhelmed leading to extensive flash flooding on small creeks and streams. Even though antecedent moisture conditions were unsea-
sonably low, major rivers in the basin rose from low flow stages (, 0.8 m) to quickly surpass flood stage. Peak stage occurred on the Bourbeuse River at Union (7.5 m) and the Big River at Byrnesville (6.8 m) within 20 h, and on the Meramec River at Eureka (8.4 m) within 31 h of the onset of the storm. Recession of the floodwater was equally rapid with all three rivers dropping below flood stage within 54 h of the initial rise. Discharge hydrographs for each of the rivers are highly symmetrical with steep recession limbs (Fig. 2). This behavior contrasts with the highly asymmetrical hydrograph that is typical of storm hydrographs and results from the short lag time between precipitation and runoff and also from the proximity of the gauging stations to the storm area (Leopold, 1994). Such hydrographs are characteristic of urban runoff or flash flood events in arid environments and this type of response has been observed for another high volume rainfall event in this basin (Waite and Alexander, 1987). The hydrographs begin to assume the more typical recession pattern only after discharge has returned to , 30% of peak flow. 1.2. Basin physiography and geochemistry The Meramec basin (10,300 km2) is located on the northeastern flank of the Salem Plateau in east central Missouri (Fenneman, 1938). This region includes foothills of the Ozark Mountains and is highly
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dissected with steep valleys and incised stream channels. Relief is greatest in the south and gradually decreases northward into the rolling hills of the Bourbeuse River sub-basin. The unconfined Ozark aquifer crops out throughout the area and consists of lower Paleozoic dolomite and limestone underlying thin soils (Imes and Emmett, 1994). Dissolution features including many springs, losing (influent) streams, ‘swallow holes’, and sinkholes allow rapid connection between surface and groundwater reservoirs (Vandike, 1995). Recharge occurs exclusively through infiltration of rainwater, with annual precipitation averaging , 96 cm and monthly totals for April and May usually exceeding 10 cm. The Meramec River and its two main tributaries, the Bourbeuse and Big rivers, drain the basin flowing generally north and east until joining the Mississippi River south of St Louis (Fig. 1). Regular surface and groundwater sampling has been carried out in the Meramec basin since 1995 (Frederickson and Criss, 1999). An extensive database of physical, isotopic and chemical measurements has been compiled (Winston, 2002) to characterize regional water chemistry and to provide a baseline against which individual rainfall events can be compared. Surface water chemistry is influenced by interactions between incoming rainwater and the soil and bedrock substrates. Average precipitation in the region is dilute (conductivity 14 mS cm21) and mildly acidic (pH 4.9). Groundwater chemistry in the basin is controlled by dissolution of carbonate host rocks in the underlying aquifers and is dominated by calcium and bicarbonate (Imes and Davis, 1991).
determined with an Oakton pHTestr 3, and turbidity was measured with a Hach 2100P turbidimeter. Aliquots for oxygen isotope analysis were transferred to borosilicate glass bottles in the field and sealed with minimal headspace. Oxygen isotope ratios were measured with a Finnigan Mat 252 IRMS using the CO2 equilibration technique of Epstein and Mayeda (1953) and are reported in the usual manner as per mil (‰) deviations from SMOW (Craig, 1961); precision is better than ^ 0.1‰. Bulk samples for dissolved ion determinations were transferred to pre-washed 500 ml HDPE bottles, refrigerated (on ice), and returned to Washington University for filtration and acidification prior to analysis. All bulk samples were processed within 24 h to remove particulates and to stabilize compositions prior to major ion analysis. Samples were drawn under vacuum through 0.2 mm nylon filter paper and then 30 ml individual aliquots for anion and cation analyses were decanted into clean, acid-washed HDPE bottles. Cation samples were acidified with nitric acid to pH ø 3.0 to inhibit carbonate complex formation, and then stored at room temperature until analysis; anion samples were not acidified but were stored under refrigeration. Concentrations of dissolved major anions and cations were determined separately using liquid ion chromatography. All samples were analyzed in triplicate and the results were averaged. Standard deviations among replicates were , 0.1 ppm (, 0.05 ppm for K) and accuracy as determined using laboratory standards was better than ^ 5%.
2. Methods
3. Geochemical response
Numerous sampling excursions were conducted from May 6 through May 25 to track changes in water chemistry over the entire course of the flood pulse particularly during the rising limb and near the discharge peak. Water samples were collected from the mid point of the flow channel from bridges over the Meramec (15 samples), Bourbeuse (22), and Big (15) rivers. Typical water chemistry parameters were determined immediately with field instruments. Temperature and electrical conductivity were measured with a Yellow Springs Instruments (YSI) 30, pH was
Water chemistry parameters during flooding typically exhibit response patterns either directly or inversely related to the discharge curve (Walling and Foster, 1975; Criss, 2002). As flood waters rise, parameters associated with the particulate phase or with soil waters typically increase while those associated with baseflow typically decrease. These behaviors are based on different flow paths with particulate matter largely resulting from surface or shallow subsurface flow (event water) while the dissolved phase is pre-dominantly associated with
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Fig. 3. Variations in specific conductivity (solid line, solid circles), turbidity (dashed line, solid diamonds), and pH (dashed line, open circles) in rivers of the Meramec basin during the May 2000 flood event. Discharge (Q, thin dashed line) is included without scale for temporal reference. Estimated pre-storm values for the Bourbeuse River on May 6 are indicated by square symbols.
Table 1 Comparison of the initial conditions and event extremes for selected water chemistry parameters in Meramec basin rivers Parameter
Stage (m) Discharge (m3 s21) Conductivity (mS cm21) Turbidity (ntu) d 18O (‰) a
Estimated.
Meramec
Bourbeuse
Big
Pre-storm
Event extreme
Pre-storm
Event extreme
Pre-storm
Event extreme
0.6 22 399 13.6 25.7
8.4 1606 91.4 1056 23.4
0.3 2.1 275a ,10.0a 25.2a
7.5 900 34.7 1180 22.9
0.7 4.2 482 9.9 25.3
6.8 663 134.0 2440 23.6
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baseflow (pre-event water) since many of these solutes are products of water –rock interactions. Fig. 3 shows variations in specific conductivity, turbidity and pH during the storm pulse for the Meramec, Bourbeuse, and Big rivers. Discharge hydrographs (thin dashed lines) are included without scale for temporal reference. Pre-storm values for some water chemistry parameters are given in Table 1 along with the maximum or minimum value measured during the event. Specific conductivity in each of the rivers follows the typical pattern featuring a sharp decline during the rising limb, reaching minima at peak discharge, and gradually returning toward prestorm values. Turbidity variations reflect the particulate load and are directly related to discharge but have a shorter time scale, with maximum values occurring well before peak discharge. Variability of pH is minor considering that incoming precipitation in this region has an average pH of 4.9 and that most of the water originated as overland flow or shallow subsurface runoff (see Section 4). The carbonate bedrock that predominates in the basin leads to rapid buffering of incoming rainfall. 3.1. Meramec River Water samples (15) were collected from the Meramec River at the site of the Eureka gauging station (Fig. 1). This site is located downstream of the confluences with the Bourbeuse and Big Rivers and thus integrates drainage from all three sub-basins. Sampling on the afternoon of May 6 established the pre-storm baseflow condition of the basin. Specific conductivity rapidly decreased to 25% of the prestorm value, with the minimum roughly corresponding to peak flow, and then gradually recovered to values within 10% of pre-storm levels by May 15 (Fig. 3a). Turbidity increased , 75-fold from the baseflow value of 13.6 ntu to a maximum value of 1056 ntu as floodwater continued to rise. By the time the crest occurred, turbidity had attenuated to , 50% of its peak value and continued to relax to near prestorm levels by May 15. The pH decreased as floodwater rose and reached a minimum value of 7.3 on the recession limb. Average pH at this site is 8.3 and the reduction by 1.0 pH unit is significant, though normal, for floodwaters in the region (Criss et al., 2001).
3.2. Bourbeuse River Sample collection commenced from the Bourbeuse River at Union after floodwaters began to rise on the morning of May 7 (Fig. 1). Specific conductivity declined rapidly from the estimated initial value of 275 mS cm21 to the minimum value of 35 mS cm21 at peak flow (Fig. 3b). A small hump in conductivity superimposed on the minimum occurred as floodwaters rose and may indicate the flushing of solutes from the shallow groundwater system. This small hump was quickly followed by a conductivity minimum that approximately coincided with peak flow, then by gradual relaxation with a return to within 10% of the pre-flood value by May 18. Turbidity increased sharply on the rising limb, climbing from , 10 ntu to the maximum recorded value of 1362 ntu approximately 10 h prior to the flood crest, then returned to normal values in about 7 days. Values of pH measured in the Bourbeuse River average 8.2 but fell to 7.5 just after the flood crest, and subsequently returned to normal values as quickly as turbidity. A small hump observed in the pH record prior to the discharge peak corresponds to the hump in conductivity and would also be consistent with the rapid delivery of event water through the vadose zone. 3.3. Big River A pre-storm water sample was collected from the Big River at Twin Rivers Bridge (river mile 0.4, Fig. 1) at 1:00 pm on May 6, 2000. Specific conductivity and turbidity responded in similar fashion to that reported for the Meramec River but the variations were larger (Fig. 3c). The Big River sub-basin received the least amount of rainfall during this storm but recorded the largest range of conductivity (482 – 134 mS cm21) and the greatest change in turbidity (10 –2440 ntu). The range of pH values (1.0 pH unit) was comparable to those observed in the Bourbeuse and Meramec Rivers. Return to baseflow conditions for all these parameters in the Big River occurred by May 14. 3.4. Dissolved solute behavior Response patterns of major dissolved solutes were similar for each river and exhibited typical flood-
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Fig. 4. Solute concentrations in the Meramec River at Eureka. Most dissolved species (Ca, Mg, Na, Cl, and SO4) exhibit responses proportional to, and in phase with, conductivity, rapidly decreasing to minima near the discharge peak and slowly returning to pre-storm values. Potassium concentration (K) increases with rising water and reaches the maximum value prior to the flood peak, behavior consistent with event water flushing of soil and shallow subsurface reservoirs. Anion data are not available for the rising limb.
related behavior. Results for samples collected from the Meramec River at Eureka are shown in Fig. 4. Prestorm (baseflow) values and event extremes are listed in Table 2. Conductivity reflects the collective behavior of individual solutes and thus the concentrations of most ions (Ca, Mg, Na, Cl, and SO4) are positively correlated with conductivity and rapidly dropped to minimum values before gradually returning to pre-storm levels. These solutes predominantly derive from water –rock interactions (Hem, 1985) and decreased concentrations are due to the mixing of baseflow with relatively dilute event water. In
contrast, potassium concentration increased with discharge and reached maximum values prior to, or coincident with, peak discharge. This response is consistent with flushing of the soil layer and shallow subsurface that would accompany the initial pulse of storm flow. Although the concentrations of most solutes became quite low during the event, the dissolved load increased for all constituents. Relative to prestorm values, sodium load increased 145%, calcium, magnesium, chloride and sulfate loads increased 1200 – 2800%. The largest increase (17,000%)
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Table 2 Solute concentrations (ppm) and loads (kg s21) in the Meramec River at Eureka and the percentage change from pre-event values Solute Ca Conc. (ppm) Load (kg s21)
Pre-storm
Event extreme
Change (%)
40 0.78
8.5 15.0
279 1823
Mg Conc. (ppm) Load (kg s21)
23 0.45
4 5.8
283 1189
K Conc. (ppm) Load (kg s21)
1.4 0.03
3 4.7
114 17,307
Na Conc. (ppm) Load (kg s21)
6 0.11
1.5 0.3
275 145
Cl Conc. (ppm) Load (kg s21)
10 0.19
3.5 5.5
265 2795
SO4 Conc. (ppm) Load (kg s21)
20 0.38
7 11.0
265 2795
occurred for potassium due to the positive correlation with discharge (Table 2). Load for each ion increased in the Meramec River with the rising limb of the hydrograph. Maximum values of calcium and sodium were observed prior to peak discharge, while magnesium and potassium reached maximum load coincident with peak discharge. The observed maximums for chloride and sulfide also occur in the sample collected near the peak discharge but due to the lack of a rising limb anion sample it was difficult to draw more detailed conclusions for the anions. Return to near pre-storm load values for all solutes occurred in concert with decreasing river discharge.
4. Hydrograph separation Oxygen isotope ratios are used to distinguish between event water (surface flow) and pre-event water (baseflow) using the method developed by Sklash and Farvolden (1979). Application of this method is possible when precipitation can be characterized by a single isotopic value, baseflow and groundwater are isotopically consistent, and no other sources confuse the mixing calculations.
Additionally, there must be a significant difference between the d 18O values of incoming precipitation and baseflow. Real-time stage and discharge readings were acquired over the internet (Mason and Yorke, 1997) for gauging stations on the Bourbeuse River at Union, the Meramec River at Eureka, and the Big River at Byrnesville. Stage measurements indicate these rivers had been at stable, low flow levels (, 0.8 m) for several weeks, supporting the assertion that the rivers were at baseflow condition before the storm. These low flow conditions were due to uncharacteristically low rainfall during the previous month. Rainfall during the month of April was less than 35% of normal and no significant precipitation had occurred at Union since April 23– 24 when 1.2 cm fell. These factors suggest that antecedent moisture conditions and vadose storage were minimal and that the isotopic and chemical composition of the rivers reflect the baseflow. Any soil moisture contribution to the storm pulse is assumed to be minor due to the baseflow condition of the rivers, low prior rainfall, and the extreme size of the precipitation event. The isotopic content of rainfall during this event did vary geographically, with rainfall becoming progressively heavier away from the storm center. The lowest d 18O value for precipitation (2 3.4‰) was at Union where the highest rainfall occurred (39.9 cm). St Clair is south of Union and received 25.4 cm of 2 2.8‰ rainfall, while at Eureka, the 11.9 cm of rainfall had a d 18O value of 2 3.0‰. In Ladue, 10.2 cm of 2 2.4‰ rain was collected, and further east in Madison County, Illinois 4.1 cm of 2 1.7‰ precipitation fell, although these two areas are not in the Meramec basin. Overall rainfall amounts and intensity during this 13 h event were extremely heavy and localized with 12.7 –39.9 cm of rain covering a 4100 km2 region (Fig. 1; Winston and Criss, 2002b). Delivery of such a large volume of rainfall over a short period effectively ensures that any temporal variability in the d 18O value of incoming precipitation would be quickly integrated. Moreover, the observed spatial variation in d 18O for rainfall within the basin (, 0.6‰) is less than the difference between the rain and baseflow (, 2‰). To account for the geographical variation, only rainfall samples collected within the Meramec basin are used to calculate a weighted average of 2 3.0‰ for
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Fig. 5. Variations in the d 18O value of flood waters in the Meramec, Bourbeuse and Big rivers during the May 2000 event, hydrographs (small dashes, right scale) are included for temporal reference. The d 18O curves (solid lines) were obtained by interpolation between individual samples (open circles) and these values are used in the calculation of the baseflow fraction (Eq. (2)). The weighted average of precipitation (PPT) across the basin was 23.0‰ and all the rivers approach this value during flooding. The Bourbeuse River actually exceeds the weighted d 18O value of precipitation indicating possible contribution of more enriched precipitation from upstream. The Meramec and Big rivers return to pre-storm values but the Bourbeuse River exhibits a shift in baseflow d 18O value.
precipitation during this event and this value is used in the hydrograph separations for the Meramec and Big rivers. A slightly heavier value (2 2.9‰) is used for the Bourbeuse River separation because three successive river samples exceeded 2 3.0‰. The d 18O variations in the rivers correlate negatively with specific conductivity, implying two component mixing between a low conductivity, high
d 18O member (precipitation or event water) and a high conductivity, low d 18O member (baseflow or pre-event water). The relationship for two-endmember mixing takes the familiar form: QT CT ¼ Q1 C1 þ Q2 C2
ð1Þ
where Q is discharge, C is the concentration of any conservative parameter, and the subscripts represent
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Fig. 6. Hydrograph separations and d 18O values for the Meramec, Bourbeuse and Big rivers. Total discharge values are from gauging station data, baseflow and event water contributions were calculated using the baseflow fraction (X ) obtained from Eq. (2). The dominance of event water during peak flooding is atypical for this region and indicates the remarkable impact of this storm.
the total concentration and the concentrations in the two endmembers. Due to the conservative nature of oxygen isotopes, Eq. (1) can be solved to obtain the fraction X of discharge derived from baseflow. Using oxygen isotopes as an example (Sklash and Farvolden, 1979), the result is: XBaseflow ¼
d18 ORiver 2 d18 OPPT 18 Baseflow 2 d OPPT
d18 O
ð2Þ
The d 18O values from individual river samples were
interpolated to provide a continuous d 18O curve that allows calculation of the baseflow fraction at all discharges (Fig. 5). Baseflow d 18O values for the Meramec (2 5.7‰) and Big Rivers (2 5.3‰) were determined from samples taken prior to the storm on the afternoon of May 6 and were assumed to remain constant since the rivers returned to these values after the event. The initial d 18O value of Bourbeuse River baseflow (2 5.2‰) was estimated from regression relationships established between this river and the
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Meramec and Big rivers. After the flood recession, Bourbeuse River baseflow stabilized at a new d 18O value (2 4.7‰) and thus a linear shift between these values has been computed to account for the change in baseflow value during the course of the event. Baseflow d 18O values are used in Eq. (2) along with precipitation and river values to calculate the baseflow fraction which is then multiplied by total discharge to obtain continuous values for baseflow discharge. Event water derived discharge is obtained by difference between the total discharge and the calculated baseflow discharge. Error analysis following the method of Genereux (1998) has been performed to identify uncertainty in the calculated pre-event fraction for each of the rivers investigated. Uncertainties in the pre-event and individual stream samples were established by analytical error in our lab (^ 0.1‰) since each of these values is represented by a single sample. Uncertainty in the event water was calculated from the standard deviation of rainfall samples (^ 0.25‰) collected in the basin and the 70% confidence interval (2 degrees of freedom). The analysis was performed for each of the rivers and calculated uncertainty values ranged from 3 to 16% over the course of the storm pulse. The uncertainty in our hydrograph separation can also be evaluated by comparing results obtained with isotopes to those calculated with other tracers. For example, specific conductivity in the Meramec River at Eureka varied from a pre-storm value of , 400 to , 90 mS cm21 at the event extreme. Assuming a low conductivity (, 25 mS cm21) endmember, we calculate an event water contribution of , 80 ^ 3% in close agreement to our isotopically derived value (80%). Given our extensive experience with isotopic studies in this basin, and the lack of a fully characterized event water chemical composition, we focus on the isotopic method below. Hydrograph separations for the Meramec, Bourbeuse and Big rivers reveal the dramatic contribution of event water during this flood event (Fig. 6). Event water increased with rising flood water, ultimately dominating the baseflow contribution during most of the flood pulse. On each river, the maximum instantaneous event water component constituted at least 70% of total discharge, and on the Bourbeuse River the baseflow contribution was negligible for at least 24 h. These results are highly unusual for
159
flooding in this region and represent the largest event water contribution reported in the literature. The isotopic response of the Meramec River at Eureka (Fig. 6a) represents an integration of storm effects over the effected portion of the basin given the location of this station near the basin outlet. Event water contributions roughly followed the overall shape of the discharge curve while baseflow increased rapidly and then remained relatively flat during the flood peak. Surface flow reached a maximum value at 80% of total discharge approximately 24 h after peak discharge, rapidly decreased to , 20% on May 12, and then gradually attenuated to 0% by May 19. Hydrograph separation and d 18O values for the Bourbeuse River are shown in Fig. 6b. As discharge increased, the d 18O value also increased, ultimately exceeding (by 0.1‰) that of the weighted rainfall average as the river crested. Baseflow and event water both increased initially but the event contribution rapidly dominated as rainfall intensity outpaced infiltration capacity. Surface runoff to the Bourbeuse River resulted in the largest event water component of all the rivers as storm water made up , 100% of flow for more than 24 h. As river stage relaxed toward prestorm levels, d 18O values slowly decreased but did not return to initial values. In fact, d 18O in the Bourbeuse River would not reach values below 2 5.0‰ until near the end of the year. The Big River received the least amount of total rainfall, exhibited the lowest d 18O shift and the most rapid recovery (Fig. 6c). Both baseflow and event water components were symmetrical peaks that rose and fell concomitantly. The baseflow contribution decreased quickly to a minimum of , 30% of total discharge, just prior to the flood peak, and then began to recover as water began to recede. By May 10 baseflow was again the dominant flow component and by May 17 the river had returned to baseflow conditions.
5. Discussion Storm pulses in the Meramec basin typically exhibit steep rising limbs followed by gradual recessions that decrease inversely with time (Criss, 1999). In the May event, the symmetric nature of the hydrographs (Fig. 2) indicates a rapid rise and fall of
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Fig. 7. Results of dissolved load calculations for major cations in the Meramec River at Eureka. Baseflow (thin short dashes) and event water (thin solid line) discharges are included for comparison. Total load (thick long dashes) increases substantially during the storm pulse but decreases along with discharge to quickly return to near pre-storm values. Baseflow loads (thick short dashes) increase along with baseflow discharge and contribute the majority of Ca, Na, and Mg at peak river discharge. Event water loads (thick solid line) for Ca and Na indicate an early flushing of these solutes that causes maximum total loads to precede the discharge peak, total potassium load is almost entirely made up of the event water component. The calculated magnesium baseflow load exceeds total load and indicates that the baseflow concentration may be less than expected or varies during the course of the event.
water characteristic of flash flooding. Flood waters in this region generally consist of 50% or more baseflow as determined by isotopic hydrograph separation (Frederickson and Criss, 1999), but during this extraordinary flood, event water comprised at least 70% of peak flow in each river. Isotopic and geochemical studies of less intense storm events in many areas typically show that baseflow comprises more than 50% of total discharge (Pinder and Jones, 1969; Sklash and Farvolden, 1979). Caissie et al. (1996) studied storm pulses in a small Canadian drainage basin and obtained increasing
event water contributions for higher intensity storm events with greater resultant discharges, the greatest being 45% during a pulse with a maximum discharge of 18.5 m3 s21. Other studies have also reported greater event water contributions for higher rainfall intensity and peak discharge events (Hooper and Shoemaker, 1986; Brown et al., 1999). The lowest peak discharge recorded during the May 2000 event was 663 m3 s21 on the Big River which also had the lowest maximum event water component at 70%. Peak discharge and event water contributions were higher on the Bourbeuse (900 m3 s21, 100%) and
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Meramec Rivers (1606 m3 s21, 80%). The larger event water contribution on the Bourbeuse River is likely due to the proximity of the gauging station to the storm center and the most extreme rainfall intensity. The baseflow fraction obtained through the hydrograph separation technique is a useful tool to examine the contribution of individual flow components to overall geochemical behavior. In most storm flow studies, baseflow is assumed to have a constant isotopic and geochemical composition. The instantaneous load (mg s21) of any solute can be calculated from the familiar expression: Load ¼ Cs Q
ð3Þ
where Cs is the solute concentration in mg l21 and Q is the discharge in L s21. Total load is calculated using individual samples and corresponding discharge, baseflow load is then calculated from prestorm solute concentration and baseflow discharge values obtained from the hydrograph separations. The baseflow component is then subtracted from total load to give the load associated with event water discharge. Loads for some dissolved ions in the Meramec River have been calculated from the concentrations given in Fig. 4 and appear in Fig. 7, baseflow and event water hydrographs are included for comparison. Total load for calcium and sodium reach maximum values well before peak discharge and coincide with maxima in event water loads (Fig. 7a and b). The load derived from baseflow is greatest at roughly the same time as peak discharge due to the corresponding maximum in baseflow discharge. Event water contributions of calcium and sodium return to relatively low levels before peak discharge, implying that mobile fractions of these solutes are easily flushed from the soil and shallow subsurface. Results from the potassium calculation support the conclusion that event water is the major source for this solute; the contribution from baseflow is generally flat while the event derived load makes up most of the total load (Fig. 7c). The magnesium load calculated from baseflow almost always exceeds the total load calculated from measured values (Fig. 7d) and indicates that either the baseflow value is too high or that the concentration of this solute in baseflow is not constant. Conductivity has been used to separate hydrographs into event water and baseflow components
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(Caissie et al., 1996; Laudon and Slaymaker, 1997). If we assume that baseflow predominates when conductivity has returned to pre-storm values, we can then assume that the d 18O value at that time also represents baseflow conditions. On the Meramec and Big rivers, conductivity and d 18O have both returned to initial values by May 17 indicating that the storm did not have a major effect on the average d 18O value of baseflow in these rivers. Conductivity returned to near initial values on the Bourbeuse River by May 25, but d 18O had only decreased to 2 4.7‰. This shift (þ 0.5‰) from the pre-storm value illustrates the impact of this storm on the d 18O value of baseflow in the Bourbeuse sub-basin. Seasonal shifting of surface water d 18O values has been observed in other studies and is driven by annual cycles in the d 18O value of incoming precipitation and other factors (Frederickson and Criss, 1999; Winston and Criss, 2002a). The d 18O value in the Bourbeuse River averages 2 5.2‰ cycling between lighter values during winter months and heavier values in the summer. This single event delivered , 30% of annual basin rainfall, shifting the d 18O value of baseflow from near the average value into the summer range. The impact is not as dramatic on the Meramec or Big rivers due to the distribution of less intense rainfall over a relatively smaller portion of their basins.
6. Conclusion A remarkable high-intensity rainfall event over east central Missouri on May 7, 2000 provided a unique opportunity to apply hydrograph separation techniques to study hydrologic response in the Meramec River basin. This event represents the highest volume and most intense rainfall reported for a hydrograph separation study. Infiltration capacity in the lower basin was rapidly overwhelmed leading to rapid runoff of event water (through overland flow and shallow subsurface pathways), and resulted in localized flash floods and regional flooding. Hydrograph separations using oxygen isotopes reveal that most (70 –100%) of the floodwater during peak flow in the lower Meramec, Bourbeuse and Big rivers originated from event water. The result is atypical for a region where limestone dissolution features facilitate rapid connection between surface
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and groundwater reservoirs and where event water usually accounts for less than 50% of flood discharge. Water chemistry followed typical flood response patterns with conductivity decreasing and turbidity increasing in the rising waters. Solute concentrations varied in concert with conductivity for calcium, magnesium, sodium, chloride and sulfate, while potassium increased with the flood waters. Load calculations reveal the dominance of the baseflow fraction as a source for dissolved Ca, Mg and Na; K is primarily associated with the event water contribution.
Acknowledgments This paper has been improved by the thoughtful comments of reviewers Bariac, Foster, Laudon, and anonymous and we sincerely thank them for their time. We also thank Everett Shock and Chris Frederickson for valuable discussion, Lloyd Waite (USGS) for assistance correcting hydrologic data, Barbara Winston for assistance during sampling trips, and Rachel Lindvall and Natalya Zolotova for analytical assistance. Supported by the NSF Hydrology Program.
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