The accumulation of excess phosphorus (P) in freshwater ecosystems can increase eutrophication, resulting in harmful cyanobacteria blooms that impair ecosystem services and human health. In many large rivers, sediment bound-P accumulates in slow-flowing areas, creating large sediment beds which can contribute to eutrophication under certain conditions. The potential for P exchange between sediment and dissolved pools is shaped by river physicochemistry. Low soluble reactive phosphorus (SRP) and dissolved oxygen (DO) concentrations can enhance sediment P release, particularly when P is bound with redox-sensitive iron and aluminum compounds. Since sediment P release can undermine expensive efforts to limit P losses from agricultural or urban areas, it is important to understand the potential for river sediment to contribute to eutrophication. We examined the potential for sediment P release in a slow-flowing section of the Maumee River between Defiance, OH and Grand Rapids and Providence Dams. Maumee River P exports are the primary cause of harmful cyanobacteria blooms in Lake Erie. At four sites, we monitored DO using a sensor positioned near the sediment-water interface (SWI) and 0.5 m below the surface; we complemented those high-frequency measurements with spatial DO surveys. Monthly, between May and September, we measured benthic sediment characteristics including P fractions, P saturation, sediment P release rates, and other aspects of sediment composition. At all sites, sediment was saturated with P increasing the risk of P release. While the majority of P was associated with a tightly-bound residual fraction, the second most common P fraction was P bound with iron and aluminum which could be released under low dissolved oxygen concentrations. During the mid-summer, SRP concentrations declined while algal biomass increased; hypoxia was common near the SWI. Thus, our results suggest that the risk of sediment P release within this section of the Maumee River is high. Sediment P release would increase the potential for eutrophication within the Maumee River and, potentially, the export of bioavailable P downstream to Lake Erie where it could enhance or sustain harmful cyanobacteria blooms.