Streambank Legacy Sediment Contributions to Suspended Sediment-Bound Nutrient Yields from a Mid-Atlantic, Piedmont Watershed

While streambank legacy sediment has been increasingly recognized as an important contributor of fine sediment to watersheds, little is known about its contributions to suspended sediment-bound nutrients. This uncertainty is exacerbated by the ubiquitous presence of legacy sediments in valley-bottoms of the Mid-Atlantic region of the United States. Using a sediment fingerprinting model, we quantified suspended sediment and sediment-bound nutrient sources for a 114 km(2) agricultural, Piedmont watershed. Sediment sources included agricultural, developed, and forested soils as well as streambanks legacy sediments. Suspended sediment and sediment-bound nitrogen (N) and phosphorus (P) concentrations were determined for 10 storms over the study period of August 1, 2017 to July 31, 2018. Streambanks contributed 50% of the annual suspended sediment yield of 1,348 kg/ha followed by agricultural (49%) and forested (1%) soils. Streambanks amounted to 26% and 32% of the suspended sediment-bound N and P exports, respectively, whereas the corresponding loads from agricultural sources were 73 and 67%, respectively. Bank contributions varied with storm magnitude and seasonality, with the highest exports associated with the largest peak flows. These results highlight that streambank sediments constitute a significant fraction of suspended sediment-bound nutrient exports and must be considered in watershed models, regulatory assessments, and management practices.