ANALYSIS AND INTERPRETATIONS OF SEDIMENT CORES FROM LAKE SEMINOLE, GEORGIA

Rivers impounded by dams experience morphological changes that create an opportunity to calculate reservoir sedimentation rates and relate them to watershed land-use history. Lake Seminole, created by the completion of the Jim Woodruff Dam in 1954, impounds the Chattahoochee and Flint Rivers. We collected 10 sediment cores (53 cm to 183 cm in length) from 5 locations in April 2023. The coring sites were chosen to sample the inundated features visible in a 2009 bathymetric survey: the Chattahoochee River channel, the Flint River channel, the confluence river channel between the two, and the adjacent floodplain surfaces. Analysis of loss on ignition (LOI) , bulk density, elemental concentrations, and short-lived radionuclide geochronology will aid in completing the following research objectives: correlating short and long sediment cores, determining whether the pre-dam sediment surface was reached at each location, measuring the sedimentation rates in the Chattahoochee and Flint arms of the reservoir, and evaluating the characteristics of each core in the context of its location and the history of Lake Seminole. The mean of the LOI from the Chattahoochee arm is 10.5% ± 3.9% and the dry bulk density is 0.630 g/cm³ ± 0.181 g/cm³. The mean of the LOI from the Flint arm is 13.5% ± 4.4% and the dry bulk density is 0.520 g/cm³ ± 0.216 g/cm³. In one core, we interpret a sharp contact between medium sand and overlying organic-rich mud at a composite depth of 135 cm to be the boundary between the pre-dam Flint River bed and reservoir sediment. This yields a sedimentation rate of 2 cm/year at that location. It is expected that sediment deposition is dominant on the Chattahoochee arm of the reservoir, in agreement with a 2011 sedimentation analysis of the reservoir conducted by the Army Corps of Engineers. We expect that contrasts in LOI, bulk density, and elemental concentrations observed in the floodplain cores similarly sample the pre-dam surface and underlying soil. These interpretations will be tested via ²¹⁰Pb, ¹³⁷Cs, and ²⁴¹Am geochronology on one or more cores. Additionally, this research will explore how differences in river management and land use in the watersheds have contributed to sedimentation differences in the Chattahoochee and Flint Rivers.