Rates and Processes of Sedimentation In Two Usace Reservoirs

Reservoir sedimentation is a critical issue because it reduces water storage capacity and downstream delivery of sediment to coastal environments. Aggradation of sand and gravel deltas at the upstream ends of impoundments, a common aspect of reservoir sedimentation, has implications for flood risk and infrastructure. These deltas are also interesting for understanding the processes of fluvial response to base-level rise. We study sedimentation in two USACE reservoirs: Lewis and Clark Lake in South Dakota and Nebraska, and Lake Seminole in Georgia and Florida. Both are the result of dams built in the mid-1950s and both exhibit active progradation of deltas since that time. Based on analysis of orthorectified aerial photographs, the Missouri River delta in Lewis and Clark Lake prograded at rates of 0.32 km^2/year in 1973-1993 and 0.42 km^2/year in 1993-2018. Much of the sediment supply for this landform comes from the sand-bedded Niobrara River, which joins the Missouri River just upstream of the reservoir. Based on analysis of USACE surveys, base level has risen at the mouth of the Niobrara River at 30 mm/year, with deposition of an alluvial fan and infilling of the valley. Measurable aggradation reaches at least 14 km upstream from the mouth, highlighting the long distance that the effects of dams can propagate into river networks. On Lake Seminole, we investigate sedimentation rates and processes in two arms of the reservoir that correspond to the two major tributaries of the Apalachicola River. The mouth of the Chattahoochee River has a large subaerial delta composed of sand, with two active lobes at present. Sediment cores collected in April 2022 capture the transition from bottomset mud to foreset sand as the delta front progrades. The Flint River does not show an obvious delta in Lake Seminole, and has an apparently lower sedimentation rate. In our ongoing work, we will be connecting reservoir sedimentation, measured through analysis of cores and remote-sensing data, with land-use and river-management characteristics of the watersheds. In particular, we are modeling sediment transport on the Chattahoochee River, as modulated by a series of dams.