“Legacy phosphorus” is the accumulation of phosphorus (P) in soils and sediments due to past inputs from fertilizer, manure, urban runoff, and wastewater. The release of this P from where it is stored in the landscape can cause poor water quality. Here, we examined whether legacy P is being released from stream and river channels in summer across a large number of watersheds, and we examined what factors (such as climate, land use, and soil types) might be driving that release.