Primary Research Areas in Hydrology:

     • Impacts of dam

     • Isotopic methods for tracing fluvial processes

     • Transport and fate of arsenic and other metals

Impacts of Dams

Flow and sediment regimes at tributary junctions on a regulated river: impact on sediment residence time and benthic macroinvertebrate communities

Abstract     Tributaries may either ameliorate or exacerbate the geomorphic and ecologic impacts of flow regulation by altering the flux of water and sediment into the flow-regulated mainstem. To capture the effects of tributary influences on a flow regulated river, long-term discharge and cross-sectional data are used to assess the geomorphic and hydrologic impacts of impoundment. In addition, the use of the short-lived cosmogenic radioisotope 7 Be (half-life 53.4 days) to link sediment transport dynamics to benthic macroinvertebrate community structure is evaluated. It is found that the 7 Be activity of transitional bed load sediment is highly seasonal and reflects both variations in activity of sediment sources and limited sediment residence time within the junction. Benthic communities also exhibit a strong seasonal variability. In the spring, neither the 7 Be activity of the sediment, nor benthic communities exhibit clear relationships with sample site location. In contrast, during the late summer the ratio of Ephemeroptera (mayflies)/Trichoptera (caddisflies) decreased significantly below tributary junctions. This decrease in benthic community ratio was driven by increases in caddisfly abundance and was strongly correlated with the presence of recently 7 Be tagged transitional bedload sediment. These observations  are probably associated with the presence of coarse, stable, and unembedded substrate downstream of tributaries and the rapid turnover of sediment that may also be associated with a rapid flux in nutrients or seston. The results show that tributaries are impacting the flow-regulated mainstem and that these impacts are reflected in the benthic community structure and in the 7 Be activity of transitional bed load sediment. Moreover, the observed reduction in competence and capacity of the mainstem following flood control suggests that these spatial discontinuities may be a consequence of impoundment.

Svendsen, K.M., Renshaw, C.E., Magilligan, F.J., Nislow, K.H., Kaste, J.M., Flow and sediment regimes at tributary junctions on a regulated river: impact on sediment residence time and benthic macroinvertebrate communities, Hydrologic Processes, 23(2):284-296, 2009.

Assessment of methods for measuring embeddedness: application to sedimentation in flow regulated streams

Abstract     Five commonly used methods for measuring embeddedness – the degree to which fine particles surround coarse substrate on the surface of the streambed – are assessed and used to evaluate the sedimentation pattern resulting from impoundment on tributaries of the Connecticut River. Results show that the EPA method best reflects the sediment regime on these rivers. On the Ompompanoosuc River, regulated by a run-of-the-river/flood control dam, embeddedness increases significantly directly downstream of the dam. On the unregulated White River, no downstream trends in embeddedness are observed. EPA results on the Ompompanoosuc River reflect the movement of a local decrease in embeddedness, interpreted as a moving region of scour, with a calculated transport rate of ~5-25 m/day. Observed transport rates are similar to previously measured sediment transport rates and consistent with results from a multi-fraction sediment transport model. Application of the EPA method to an additional regulated tributary demonstrates the effects of dam management on embeddedness. Flow regulation with high sediment trapping efficiency results in a decrease in embeddedness downstream of the dam.  Results provide insight into the utility of available methods for evaluating the effects of management practice on streambed composition.

Sennatt, K.M., Salant, N.L., Renshaw, C.E., Magilligan, F.J., Assessment of methods for measuring embeddedness with application to sedimentation in flow-regulated streams. Journal of the American Water Resources Association, 42(6):1671-1682, 2006.

Short and long-term changes to bed mobility and bed composition under altered sediment regimes 

Abstract     Altered flow and sediment transport regimes from impoundment can result in significant channel bed composition changes that exacerbate the geomorphic and ecological effects of flow regulation. Using long-term discharge and cross-sectional data in combination with a two-fraction sediment transport model, we assess changes in the downstream bed of two flow-regulated rivers with equivalent dam-induced changes in flow but opposite changes in sediment flux. Supply limitation has led to incision and armoring in one case while supply excess has led to aggradation and embeddedness in the other. Under limited sediment supply, bed elevation variability decreases soon after impoundment, while excess sediment supply results in a decades-long gradual decrease in both bed elevation variability and depth of incision. Although the balance of sediment supply and transport differs between dam management styles, in both cases both the immediate and more gradual changes can be explained within the framework of a two-fraction sediment transport model. Our results demonstrate the importance of considering bed composition on sediment transport predictions and the development of management strategies for ecosystem maintenance.

Salant, N.L., Renshaw, C.E., Magilligan, F.J., Short and long-term changes to bed mobility and composition under altered sediment regimes, Geomorphology, 76, 43-53, 2006.

Isotopic Methods for Tracing Fluvial Processes

Evaluating the impacts of impoundment on sediment transport using short-lived fallout nuclides

Abstract     Dams greatly influence water and sediment discharge regimes and can have significant impacts on channel morphology and sediment storage. By using the short-lived fallout radionuclide 7Be (t_1/2 = 53.4 days) as a tracer of fine (~0.25-2 mm) bed-load transport, we capture the sedimentological and geomorphic impacts of the Union Village Dam, located on the Ompom¬panoosuc River, in eastern Vermont, USA. We measured 7Be activities in approxi¬mately monthly samples from streambed sediments in a regulated stream and an unregulated control stream. In the regulated stream our sampling spanned an array of management conditions during the annual transition from flood control in the winter and early spring to run-of-the-river operation from late spring to autumn. Because sediment stored behind the dam during the winter quickly became depleted in 7Be activity, it became possible to track this plug of "dead" sediment as it moved downstream. Measured average sediment transport velocities (30-80 m day-1) exceed those typically reported for bulk bed-load transport and are remarkably constant across varied flow regimes, possibly due to corresponding changes in the bed sand fraction. Results also show that the length scale of the downstream impact of this dam management (winter pool and summer run-of-the river with minimal sediment trapping efficiency) on sediment transport can be short (~1 km); beyond this distance the sediment trapped by the dam is replaced by new sediment from point bars, tributaries and other downstream sources. The benthic community structure indicates significantly greater abundance of caddisflies downstream of the dam due primarily to a lack of bed disturbance following impoundment.

Magilligan F.J., Salant, N.L., Renshaw, C.E., Nislow, K.H., Heimsath, A., and Kaste, J., , Evaluating the impacts of impoundment on sediment transport using short-lived fallout radionuclides, In: Sediment Dynamics and The Hydromorphology of Fluvial Systems (Ed. Rowan, J. and Werrity, A.), The International Association of Hydrological Sciences (IAHS) Special Publication 306, IAHS Press, Wallingford, UK, 2006.

The use of short-lived radionuclides to quantify transitional bed material transport in a regulated river

Abstract     We investigate the use of the short-lived fallout radionuclide beryllium-7 (Be; t_1/2= 53.4 days) as a tracer of medium and coarse sand (0·25–2mm), which transitions between transport in suspension and as bed load, and evaluate the effects of impoundment on seasonal and spatial variations in bed sedimentation. We measure 7Be activities in approximately monthly samples from point bar and streambed sediments in one unregulated and one regulated stream. In the regulated stream our sampling spanned an array of flow and management conditions during the annual transition from flood control in the winter and early spring to run-of-the-river operation from late spring to autumn. Sediment stored behind the dam during the winter quickly became depleted in 7Be activity. This resulted in a pulse of ‘dead’ sediment released when the dam gates were opened in the spring which could be tracked as it moved downstream. Measured average sediment transport velocities (30–80 metres per day) exceed those typically reported for bulk bed load transport and are remarkably constant across varied flow regimes, possibly due to corresponding changes in bed sand fraction. Results also show that the length scale of the downstream impact of dam management on sediment transport is short (c. 1km); beyond this distance the sediment trapped by the dam is replaced by new sediment from tributaries and other downstream sources. 

Salant, N.L., Renshaw, C.E., Magilligan, F.J., Kaste, J.M., Heimsath, A.M., Nislow, K., The use of fallout radionuclides to quantify transitional bed load transport in a regulated stream, Earth Surface Processes and Landforms, 32(4):509-524, 2007.

The use of stream flow routing for direct channel precipitation with isotopically-based hydrograph separations: the role of new water in stormflow generation

Abstract     Understanding the pathways by which event water contributes to stream stormflow provides insight into stormflow generation mechanisms. We analyze the impact of storm size on the relative contribution of event water to stormflow by using natural variations in the oxygen isotopic composition of precipitation and stream water to separate multiple stormflow hydrographs from a single fourth-order, 1212 ha catchment. We extend previous isotope-based hydrograph separations by independently accounting for the contribution of event water via direct channel precipitation to the stream hydrograph. The direct channel precipitation contribution is determined using a numerical model of stream flow routing though the catchment, taking precipitation and digital elevation data as input variables. For the range of storm sizes sampled, having recurrence intervals ranging from less than a week to , 4 months, essentially all the event water in stream stormflow can be attributed to direct channel precipitation. Event water not directly falling on the stream channel indirectly contributes to stormflow by increasing the subsurface discharge of pre-event water to the stream. Neither the hydrograph separation data, field observations during the precipitation events, nor experimental observations of flow in a large-scale natural soil column extracted from the watershed are consistent with macropore flow or groundwater ridging as the primary mechanism responsible for increasing subsurface discharge. Results from a series of artificial rain experiments using the unsaturated natural soil column are consistent with a preferential kinematic flow model and indicate that the discharge of pre-event water to the stream during a storm event may be controlled by kinematic flow processes within the watershed soils. 

Renshaw, C.E., Feng, X., Sinclair, K.J., Dums, R.H., The use of stream flow routing for direct channel precipitation with isotopically-based hydrograph separations:  The role of new water in stormflow generation, J. Hydrology, 273:205-217. 2003.

Transport and Fate of Arsenic and Other Metals

Impact of land disturbance on the fate of arsenical pesticides

Abstract     Increasing development of historic farmlands raises questions regarding the fate of pesticides that may have been applied when these land were in cultivation.  In two orchards where arsenical pesticides were applied in early twentieth century and a third control field where arsenical pesticides were never used we quantified the arsenic and lead phases and budgets in field soils and sediments in adjacent drainages. We also measured metal loads in two common macroinvertebrate taxa within the wetland at the outlet of the drainages.  We find that the applied arsenic and lead have undergone little vertical redistribution;  concentrations of arsenic and lead in the top 25 cm of contaminated orchard soils are higher than in the uncontaminated control field.  However, none of the applied lead arsenate remains in its original mineral phase.  Instead, the metals are now primarily adsorbed onto fine silt and clay-sized amorphous oxides and organic matter.  Further, physical erosion associated with tilling and replanting appears to have mobilized the fine-particulate-bound arsenic and lead in one orchard. The remobilized metals are found in sediments in the stream channel draining the tilled orchard. It is unclear if the arsenic and lead transported sediments are biologically active; average macroinvertebrate metal burdens in the wetland are not elevated above those observed elsewhere in the region. But little of the mobilized metals may have reached the wetland.  These results demonstrate that land use change can significantly impact the retention of arsenical pesticides.

Renshaw, C.E., Bostick, B.C., Feng, X., Wong, C. K., Winston, E.S., Karimi, R, Folt, C.L., Chen, C.Y.., Impact of land disturbance on the fate of arsenical pesticides in orchard soils, Journal of Environmental Quality, 35, 61-67, 2006.

Landfill-stimulated iron reduction and arsenic release at the Coakley superfund site (NH)

Abstract     Arsenic is a contaminant at more than one third of all Superfund Sites in the United States.  Frequently this contamination appears to result from geochemical processes rather than the presence of a well defined arsenic source.  Here we examine the geochemical processes that regulate arsenic levels at the Coakley Landfill Superfund Site (NH), a site contaminated with As, Cr, Pb, Ni, Zn and aromatic hydrocarbons.  Long-term field observations indicate that the concentrations of most of these contaminants have diminished as a result of treatment by monitored natural attenuation begun in 1998; however, arsenic levels increased modestly over the same interval.  We attribute this increase to the reductive release of arsenic associated with iron hydroxides.  X-ray spectroscopy indicates that arsenic is associated with ferrihydrite within a glaciomarine clay layer within the overburden underlying the former landfill.  Anaerobic batch incubations that stimulate iron reduction in the glaciomarine clay release appreciable dissolved arsenic and iron.  Field observations suggest that iron reduction associated with biodegradation of organic waste might be partly responsible for arsenic release; over the five year study period since a cap was emplaced to prevent water flow through the site, decreases in groundwater dissolved benzene concentrations at the landfill are correlated with increases in arsenic concentrations, consistent with the microbial decomposition of both benzene and reduction of arsenic-bearing iron oxides.  Treatment of contaminated groundwater increasingly is based on stimulating natural biogeochemical processes to degrade the contaminants.  These results indicate that reducing environments created within organic contaminant plumes may release arsenic. In fact, the strong correlation (>80%) between elevated arsenic levels and organic contamination in groundwater systems across the U.S. suggests that arsenic contamination caused by natural degradation of organic contaminants may be widespread.

deLemos, J.L., Bostick, B.C.,  Renshaw, C.E., Stürup, S., Feng, X., Arsenic contamination caused by groundwater remediation, Environmental Science and Technology, 40(1), 67-73, 2006.