HR: 08:45h
AN: H51F-03    [Abstracts]
TI: Characterizing The Hydro-Geomorphic Effects of Impoundment: Reference Conditions, Approaches, and Long-Term Fluvial Adjustments
AU: * Magilligan, F J
EM: magilligan@dartmouth.edu
AF: Dartmouth College, Department of Geography 6105 Fairchild, Hanover, NH 03755 United States
AU: Nislow, K H
EM: nislow@dartmouth.edu
AF: USDA Forest Service, Northeastern Research Station, Amherst, MA 01003 United States
AU: Salant, N L
EM: Nira.L.Salant@Dartmouth.EDU
AF: Dartmouth College, Earth Sciences Department 6105 Fairchild, Hanover, NH 03755 United States
AB: Predicting, or even understanding, the effects of impoundment requires knowledge of the pre-dam boundary conditions, including water and sediment discharge regimes, longitudinal and cross-sectional channel characteristics, and watershed disturbance history. To characterize pre-dam hydrologic regimes, we identified 21 gages across the U.S. from the National Inventory of Dams that met length-of-record criteria. Using a hydrologic model, the Indicators of Hydrologic Alteration, and GEV flood frequency analysis, we determined the pre- and post-dam hydrologic regimes for these sites. Results highlight changes in the extremes for high and low flows, and the importance of evaluating these effects over various flow durations. Other significant adjustments included changes in the annual hydrograph, primarily in the increased number of hydrograph reversals and significant shifts in the timing of high and low flows. Post-dam reductions in the bankfull discharge ranged from 20-95%, averaging 60%. To capture long-term geomorphic adjustments, we focus on the Upper Connecticut River watershed where numerous dams exist. For the at-a-station morphological adjustments, we used long-term field measured USGS data of channel hydraulic geometry spanning pre- and post-dam conditions. Significant adjustments occurred at cross-sectional scales, although not all of the streams responded similarly. The magnitude of hydrologic adjustment, dam type, and initial conditions control the magnitude and direction of change. The range of hydraulic and morphologic adjustments limits prescriptive management approaches as not all streams behaved similarly to impoundment, signaling the importance of disturbance history and boundary conditions. To identify the contemporary sediment discharge regime and impoundment's impact on embeddedness and sediment residence time, we use short-lived fallout radionuclides, especially, 7Be, to express event-scale sediment discharge. With this technique we are able to capture the effect of dam operation on the longitudinal transport and storage of sediment.
DE: 1803 Anthropogenic effects
DE: 1815 Erosion and sedimentation
DE: 1821 Floods
DE: 1824 Geomorphology (1625)
DE: 1857 Reservoirs (surface)
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting