Multimethod approach to estimate spatial sedimentation rates and volumes in a ~70-year-old, large reservoir

Sediment accumulation in aging reservoirs is a growing challenge for water‑resource management, yet few systems have high‑resolution, spatially distributed measurements of long-term sedimentation. Impounded in 1954 at the confluence of the Chattahoochee and Flint Rivers, Lake Seminole (152 km 2 ; Georgia–Florida, USA) offers a natural laboratory for evaluating variable sedimentation patterns, particularly due to contrasting sub-basin characteristics and an active subaerial delta solely in the Chattahoochee arm. We integrate new (n=5) and published (n=2) composite sediment cores and repeat topobathymetric cross‑sections (n=10) to characterize spatial patterns in sedimentation rates and volumes. Sediment cores from the Chattahoochee, Flint, and confluence arms were analyzed to characterize pre‑ and post-dam stratigraphy, physical sediment properties, and 137 Cs chronologies. Integrating cores and surveys, subaqueous accumulation rates in the Chattahoochee, Flint, and confluence arms ranged 0.4–11.6 cm yr -1 , 0.2–2.5 cm yr -1 , and ≤0.3 cm yr -1 , respectively. The data reveal high spatial variability, particularly in the Chattahoochee arm, with faster accumulation in the delta front and paleo-channels than adjacent paleo‑floodplains. Cross‑section-derived rates (n=40) broadly agree with core chronologies but diverge in low‑accumulation areas, underscoring the sensitivity of survey‑based estimates to depositional environments. Synthesizing rates using a Monte Carlo approach, we estimate 8.4×10 7 ±9.1×10 6 m 3 of sediment accumulated from 1954 to 2023—a lake-wide average of 1.40±0.15 cm yr⁻¹. Thus, Lake Seminole has lost 18.6±2.0% of its total storage capacity. Our results provide the most complete reconstruction of sedimentation in Lake Seminole to date and demonstrate the need for spatially explicit, multimethod assessments to inform sustainable sediment management in large reservoirs.