The use of 7 Be to quantify downstream trends in sediment transport below a flood-control dam

1. Sedimentation and sediment storage in a flow-regulated stream and the impact on aquatic ecosystems Nira L. Salant, Carl E. Renshaw, Francis J. Magilligan, James M. Kaste, Keith H. Nislow Dartmouth College: Department of Earth Sciences, Department of Geography The use of 7Be to quantify downstream trends in sediment transport below a flood-control dam Paper number:H53B-1237 Introduction Flow regulation by dams has a major impact on the hydrology, morphology and ecology of river systems. The primary purpose of this study is to link the ecological impact of a dam to the geomorphic changes caused by dam-induced changes in flow. Study Sites: Ompompanoosuc River, VT Union Village Dam Results 7Be-derived transport rates calculated based on position of “dead” and “new” plug of sediment throughout sample period Three time periods Highly regulated flow: Early Spring Partially regulated flow: Mid-spring Unregulated: Late spring • ******** • The average transport rates during the three time periods of this study are remarkably constant despite large variations in natural flows. Results from the two-fraction transport model indicate that reductions in flow during late spring and early summer are offset by increases in the sand fraction of total bed load. Given typical uncertainties in grain size distributions, it may be difficult to obtain reliable estimates of transport rates using models alone. Our results show that in some cases fallout radionuclides offer a simple and efficient method for directly measuring transport rates. • ************** The proposed approach for achieving this goal is novel in two respects. First, I will quantify dam-induced changes by measuring the length scales over which these geomorphic and ecological changes occur. Second, in addition to traditional measures of embeddedness and habitat degradation, I will investigate the use of radionuclide dating to determine geomorphic change via sediment deposition and transport. Specific objectives: 1. Link the ecological impacts of dams to the geomorphic response resulting from hydrologic changes. 2. Determine the length-scale and magnitude of these impacts. 3. Determine the residence time of stored sediment through the use of fallout radionuclides and compare those results to traditional measures of embeddedness. Significance to larger study Embeddedness values generally increase between sample dates, indicating an increase in sediment deposition. This is consistent with our interpretation of an increase in the sand fraction of total bed load with time. The ecological response to sedimentation will be the focus of continued study. Methods Regulation impact Flood control switch to run-of-the-river in spring Three time periods: three levels of regulation Sites: downstream progression of regulation 7Be Received from atmosphere when particles are exposed – “new” sediment Locked in snow during winter; under water will decay – “dead” sediment Spatial variations in sediment 7Be activity reflect differences in the time since the sediment was last exposed Samples Monthly grab-samples from streambed Timeframe captured temporal trends in flow Dried, sieved, activities counted in Ge detector Three measures of impact By linking measures of sedimentation (embeddedness), residence time (radionuclides), and biotic disturbance at multiple positions below a dam, this study will investigate the ways in which flow regulation dictates the spatio-temporal pattern of sedimentation and how sedimentation determines the ecological response Embeddedness Degree to which fine sediments surround coarse substrates on the surface of a streambed Fallout radionuclides Benthic macroinvertebrates Biotic indicators of degradation Acknowledgements Funding National Science Foundation: Geography and Regional Science Program Geological Society of America: Quaternary Geology and Geomorphology Division’s Arthur D. Howard Award Vermont Geological Society Dartmouth College Earth Sciences Department Asisstance Dartmouth College Biology Department: Craig Layne, Darren Ward U.S. Forest Service: Scott Wixsom U.S. Geological Society: Ken Toppin U.S. Army Corps of Engineers: Greg Hanlon Jeff Ojala Julie Jo Walters Kaoru Itakura Kelly Sennatt Contact: nira.salant@dartmouth.edu • Comparison to Sediment Transport Models