HR: 1340h
AN: H53B-1237    [Abstracts]
TI: The Use of Fallout Radionuclides to Quantify Downstream Trends in Sediment Transport Below a Flood-control Dam
AU: * Salant, N L
EM: Nira.Salant@Dartmouth.EDU
AF: Dartmouth College Department of Earth Sciences, HB 6105 Earth Sciences Dartmouth College , Hanover, NH 03755 United States
AU: Renshaw, C E
EM: Carl.Renshaw@Dartmouth.EDU
AF: Dartmouth College Department of Earth Sciences, HB 6105 Earth Sciences Dartmouth College , Hanover, NH 03755 United States
AU: Magilligan, F J
EM: Francis.Magilligan@Dartmouth.EDU
AF: Dartmouth College Department of Geography, HB 6017 Geography Dartmouth College, Hanover, NH 03755 United States
AU: Kaste, J M
EM: James.Kaste@Dartmouth.EDU
AF: Dartmouth College Department of Earth Sciences, HB 6105 Earth Sciences Dartmouth College , Hanover, NH 03755 United States
AB: Flow regulation by dams has a major impact on the hydrology of river systems, altering the timing, magnitude, and frequency of natural flows. Dams both store water and capture sediment, so that the downstream geomorphic effect differs whether considering the dam's effect on sediment discharge or its effect on transport capacity. While the geomorphic effects of flow regulation have been well quantified, few studies have focused on the mechanism by which these changes occur. The purpose of this study is to investigate how flow regulation alters the sediment transport regime below the dam, specifically in relation to sediment residence time, and to use short-lived fallout radionuclides to quantify this effect. Sediment residence time varies as a function of landscape, position, climate, and anthropogenic disturbance. The 7Be activity of transitory bed sediment was measured bi-weekly from February 2004 until August 2004 at multiple sites below a flood-control dam on the Ompompanoosuc River, Vermont. Sites were selected to represent a progression of flow regulation; highly regulated below the dam and progressively less regulated with distance downstream. Samples were taken following large storm events and during dry periods in order to capture temporal trends in sediment flux, mobilization, and deposition. The transition from flood control to run-of-the-river operation in late spring allowed us to compare sediment flux under regulated and natural flows. Because of the short half-life of fallout 7Be (53.4 days), the sediment stored behind the dam quickly became depleted in 7Be activity during the winter. Thus the spring release provided a pulse of effectively "dead" sediment which could be tracked as it moves downstream. This pulse of "dead" sediment modified the normal recovery of downstream sites. In addition, sediment exposed by the draining of the reservoir behind the dam provided an easily mobilized source of "new" sediment that was transported downstream. Using the location of these sediment pulses at different points in time, we were able to calculate an average sediment transport velocity and flushing rate. These results indicate that changes occur over relatively short distances and short timeframes, especially during high flows. This study has major implications for river research and management, presenting a quantitative technique for assessing the effect of flow regulation and dam operation on sediment transport and storage.
DE: 1800 HYDROLOGY
DE: 1824 Geomorphology (1625)
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting