We use a dated sediment core from Lake Whittington (USA) in the lower Mississippi River to reconstruct linkages in the carbon cycling and fluvial sediment dynamics over the past 80?years. Organic carbon (OC) sources were characterized using bulk (?13C, ramped pyrolysis-oxidation (PyrOx) 14C, ?15N, and TN:OC ratios) and compound-specific (lignin phenols and fatty acids, including ?13C and 14C of the fatty acids) analyses. Damming of the Missouri River in the 1950s, other hydrological modifications to the river, and soil conservation measures resulted in reduced net OC export, in spite of increasing OC concentrations. Decreasing ?13C values coincided with increases in ?15N, TN:OC ratios, long-chain fatty acids, and lignin-phenol concentrations, suggesting increased inputs of soil-derived OC dominated by C3 vegetation, mainly resulting from changes in farming practices and crop distribution. However, ramped PyrOx 14C showed no discernible differences downcore in thermochemical stability, indicating a limited impact on soil OC turnover.