The Illinois River receives nutrients from point and non-point runoff from upstream urban and agricultural areas. These excess nutrients could be contributing to harmful algal blooms in the Illinois River and its tributaries. Successfully mitigating the blooms requires an understanding of the underlying nutrient processes occurring in the river. Phosphorus is often the nutrient that limits phytoplankton growth and is a substantial concern in aquatic ecosystems because it can be stored long-term in the streambed sediment. Streambed sediment can act as a source or sink for phosphorus, and this dynamic is influenced by site conditions, including historic phosphorus loading and current stream water phosphorus concentrations. This study quantified streambed sediment phosphorus dynamics at 10 stream sites in the Illinois River Basin during the summer and fall of 2024.