Stream channel storage: Closing the watershed total phosphorus budget

A 3-year total phosphorus (TP) budget was created for Walnut Creek, a ∼5200 ha agricultural watershed in central Iowa. The budget, developed from measures of streambank erosion, overland flow, baseflow, and flux to floodplain storage, was created with the specific goal of estimating in-channel storage contributions to annual TP loads-a component often poorly understood and rarely quantified at the watershed scale. Remobilization of in-channel sediment-bound P storage was estimated to dominate contribution to annual P loads, with overland flow the second greatest contributor. Streambanks represented only a minor contribution to loads and were surpassed in importance by baseflow dissolved P in drought years with reduced direct runoff. Although flux to floodplain storage was negligible, due to relatively low peak flows over the study's duration, and the degree of channel incision, the floodplain would represent a substantial P storage opportunity during years with greater peak flows. High-discharge events delivered the majority of the annual P load in two of the three study years, and the impact of these events is predicted to increase with progression of channel evolution and climate-driven increases in precipitation event intensity. Restoration of watershed hydrology (e.g., reduction of peak flows) and increased P flux to long-term floodplain storage through enhancement of channel-floodplain connectivity will have the greatest impact for decreasing annual P loads.