Trace element contamination is known to be widely present in sediment of Lake Roosevelt and the riverine reach of the Columbia River in Washington State, USA due to discharges from several smelters and numerous mines dating back to the mid-1800's. In this study, the concentrations of aqueous elements in contact with bed sediment from the lake and river were examined under varying degrees of physical mixing and time scales. Contrasting geochemical processes affecting aqueous concentrations were inferred from the release of major ions (Ca and Si), elements enriched in metallurgical smelter slag (Cu and Sb), and redox-sensitive species (Fe, Mn, Mo and U). Releases of major ions reflect the contrasting sediment substrates along the length of the river and large reservoir. Calcium released from carbonate minerals and slag particles was most pronounced in regions of carbonate bedrock and near sediment deposits with a large component of slag material, while Si released from unconsolidated glacial/fluvial sediment increased with increasing distance downstream. Sb release was a consistent indicator of slag presence and weathering, possibly because its anionic nature inhibits readsorption onto metal oxides. In contrast, Cu release was quite variable, likely due to varying degrees of copper readsorption or co-precipitation onto metal oxides. The release of Mo and U appeared to be affected by redox conditions, which were assessed using aqueous Fe and Mn concentrations.