Depressional zones are ubiquitous features in Midwestern agricultural watersheds. These zones are characterized by closed drainage processes that result in the storage of sediments and sediment-bound nutrients. During large, infrequent rainfall-runoff events, however, depressional zones may act as sources of nutrients if events temporarily overwhelm infiltration capacities of soils. This research examines the source/sink dynamics in a depressional zone (~5.7 ha) in Wisconsin, focusing on phosphorus (P) that migrates through an agricultural watershed via these intermittently operative pathways of movement. Phosphorus sorption dynamics in depressional zones were examined by equilibrium P concentration (EPC) analyses, which were then compared with field data acquired using automated runoff samplers during depressional zone spillage. Mobility of P in colloidal-bound form was also examined using runoff data collected during spilling events. Permanent P storage was investigated by relating EPC analysis results to sediment erosion and deposition rates determined from fallout radionuclide (¹³⁷ Cs and ²¹⁰ Pb) activities at 0-2, 2-5, 5-10, 10-15, 15-20, 20-25, 25-30, 30-40, 40-50, and 50-60 increments in the soil profile. Depressional zones are of particular importance to nonpoint pollution science and policy, as these features act as both sources (detention) and sinks (retention), constituting an under-examined pathway of sediment-bound constituents.