The single most important factor affecting the longevity of US dams is sedimentation, reducing and perhaps eliminating the reservoir capacity for future sediment storage and flood wave attenuation. A better understanding of the sedimentation rates and sediment dynamics is required for a better management of these dams. Towards this, we collected and analyzed 7 sediment cores from Union Lake for excess 210Pb and 137Cs to determine the sediment accumulation rates (210Pbxs-based: 0.12 to 0.28 g cm− 2 y− 1; peak 137Cs-based: 0.13 to 0.29 g cm− 2 y− 1). These average sediment accumulation rates obtained using radionuclides are compared with three other methods: i) bathymetry-based, from the cumulative mass depth of the core and age of the impoundment (0.09 to 0.26 g cm− 2 y− 1); ii) sediment-yield curve obtained for 61 other watersheds (1.2 g cm− 2 y− 1); and iii) gage data of sediment discharge (0.21 g cm− 2 y− 1). Such a comparison provides insight on refining the sediment-yield curve-based sediment loading in the impoundment. Vertical profiles of 137Cs provide not only insight on sediment mixing (based on a novel method by analyzing the Gaussian-like curve), but also the effectiveness of vibra-coring method in retrieving the full length of the soft sediment deposited since the construction of the dam. The present investigation can serve as a model study for application to other dams in the Great Lakes region and has implications to larges lakes in the Great Lakes catchment for the prediction of sediment accumulation rates using a variety of methods.