Examining Sediment Accumulation Rates and Deltaic Processes in a Large Reservoir

Abstract

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 sedimentation in Lake Seminole, a 123 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 actively prograding. High-resolution topographic and bathymetric datasets and historical cross section data were analyzed to measure the evolution of this delta and characterize subaerial and subaqueous sedimentation in the reservoir more broadly. In addition, a comparative land-cover change analysis was conducted for the three watersheds that drain to Lake Seminole to explore potential links between the suspended sediment generated by these surface disturbances and sedimentation patterns in each arm of the reservoir. Across the entire reservoir body, subaqueous sediment accumulated at a rate of 0.81 cm/yr between 1957–1976 and 0.12 cm/yr between 1976–2009, with the highest sedimentation rates occurring shortly after dam construction. Within the Chattahoochee arm, subaqueous sediment accumulated at a rate of 2.79 cm/yr between 1957–1976 and 0.68 cm/yr between 1976–2009, whereas, in the Flint arm, subaqueous sedimented eroded at a rate of -0.21 cm/yr between 1957–1976 and accumulated at a rate of 0.08 cm/yr between 1976–2009. Qualitative observations indicate that the erosional signal in the Flint is focused in the upstream-most portion of the arm and a depositional signal emerges farther downstream. On the Chattahoochee River delta, subaerial sediment accumulated at a rate of 1.46 ± 0.48 cm/yr between 2007–2018, with the most rapid areas of aggradation (> 4.5 cm/yr) located around the exterior edges of the delta islands. No strong link was found between the percentage of each watershed that underwent land cover change and the sedimentation rates in each reservoir arm. This analysis will provide new insights into the physical processes of 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.