20-10 Lake Seminole: a Laboratory for Understanding Reservoir Sedimentation Rates, Processes and Impacts, Georgia and Florida, USA

Abstract:

Lake Seminole, a reservoir managed by the U.S. Army Corps of Engineers (USACE), was first impounded in 1954. The lake floods the confluence of the Chattahoochee and Flint Rivers, and these form separate arms of the reservoir. These two rivers have similar watershed sizes (~20,000 km²). The Chattahoochee River is one of the most heavily managed systems in the country, with 14 dams along its mainstem, including two with large flood-control capacity and several that are used for hydropeaking electricity generation. By contrast, the Flint River has only two mainstem dams, neither of which alter the hydrology. Furthermore, Lake Seminole is managed so that the water level remains relatively constant in time, varying ±0.3 m in elevation. These characteristics make Lake Seminole an outstanding laboratory for studying reservoir sedimentation. Strikingly, the Chattahoochee arm exhibits a sandy subaerial delta, which prograded at 0.12 km²/yr, based on aerial photographs from 1994 and 2015, and aggraded 1.5±0.5 cm/yr, based on differencing lidar surveys from 2007 and 2018. Repeat cross-section surveys by the USACE since 1957 and sediment cores collected in 2023 show sedimentation rates up to 12 cm/yr in the former channel and 0.5-3 cm/yr on the inundated floodplain of the Chattahoochee River. By contrast, the Flint arm has no subaerial delta, with sedimentation rates up to 2.5 cm/yr in the paleo-channel and 0-0.9 cm/yr across the paleo-floodplain. We speculate that these contrasting styles result from a combination of (1) greater and sandier sediment load delivered by the Chattahoochee River; (2) differing geometries of the inundated river valleys; (3) daily fluctuations in discharge due to hydropeaking; and/or (4) differing degrees of sediment trapping and altered flood hydrology by upstream dams. We explore these hypotheses with estimates of sediment provenance from Rb-Sr isotope measurements of reservoir sediments, and a two-dimensional, depth-averaged, process-based morphodynamic model. Overall, the estimated volume of reservoir sediment accumulated from 1954–2023 is 7.1×10⁷±4.7×10⁵ m³, which represents a storage capacity loss of ~0.23% per year, or ~16% of the original capacity. While this rate is modest compared to reservoirs in high-relief regions, the sedimentation is impacting river morphology and flood risk both upstream and downstream of the reservoir.

© Copyright 2026 The Geological Society of America (GSA), all rights reserved.