EXAMINING SEDIMENT ACCUMULATION RATES AND DELTAIC PROCESSES IN A LARGE RESERVOIR: LAKE SEMINOLE, APALACHICOLA-CHATTAHOOCHEE-FLINT RIVER SYSTEM, USA

Reservoir sedimentation is a significant issue not only because it limits a reservoir’s water storage capacity and threatens its ability to meet environmental and societal needs, but also because it reduces the amount of sediment reaching downstream coastal ecosystems where sediment loading sustains critical habitat for wildlife and fisheries. Reservoir deltas in particular can decrease channel capacity and lead to an increased flood risk for populations living in the alluvial plain upstream of dams; however, studies of these landforms are underrepresented in the literature. To address that knowledge gap, this project examines reservoir delta sedimentation in the U.S. Army Corps of Engineers-owned Lake Seminole, a 152 km² surface-water impoundment created in 1954 and located at the junction of the Chattahoochee and Flint Rivers in Florida and Georgia. Where it enters Lake Seminole, the Chattahoochee River has a large subaerial delta that is presently exhibiting active progradation. We analyzed high-resolution topographic and bathymetric datasets and historical cross section data to measure the evolution of this delta and characterize sedimentation in the reservoir and upstream river corridors more broadly. This work was supplemented by modeling delta progradation on the Chattahoochee River using a 1D finite difference formulation for river response to base level rise. Based on five cross sections that sample the Chattahoochee River delta, the average sedimentation rate was 1.50 cm/year between 1957–2009, with the highest sedimentation rate (2.84 cm/year) occurring from 1957–1976, shortly after dam construction. These estimates are averages over the entire width of each cross section. Qualitative observations indicate that sedimentation is focused in the former Chattahoochee River channel. In addition, a preliminary difference of lidar surveys has shown that sediment accumulated at a rate of about 13 cm/year from 2007–2018 at the edges of the subaerial delta islands. Ultimately, this study aims to advance new insights into the physical processes controlling reservoir sedimentation that can be used to inform river management practices and decrease the negative impacts of sediment trapping not only within reservoirs, but also upstream and downstream of dams.