Impact of an Extreme Storm Event on River Corridor Bank Erosion and Phosphorus Mobilization in a Mountainous Watershed in the Northeastern United States

Movement of sediment, and associated phosphorus, from stream banks to freshwater lakes is predicted to increase with greater frequency of extreme precipitation events. This higher phosphorus load may accelerate harmful algal blooms in affected water bodies, such as Lake Champlain in Vermont, New York, and Québec. In the Mad River, a subwatershed in central Vermont's Lake Champlain Basin, extreme flooding from Tropical Storm Irene in 2011 caused extensive erosion. We measured stream channel change along the main stem between 2008 and 2011 by digitizing available prestorm and poststorm aerial imagery. Soils were sampled post Irene at six active stream erosion sites, using an experimental design to measure differences in soil texture and phosphorus both with depth (90 cm) and distance from the stream. In addition to total phosphorus (TP), we determined bioavailable (soil test) phosphorus (STP) and the degree of phosphorus saturation (DPS). The six sites represented a 0.87‐km length of stream bank that contributed an estimated 17.6 × 103 Mg of sediment and 15.8 Mg of TP, roughly the same as average annual watershed export estimates. At four sites, the STP and DPS were low and suggested little potential for short‐term phosphorus release. At two agricultural sites where the lateral extent of erosion was high, imagery showed a clear loss of well‐established riparian buffer. Present‐day near‐stream soils were elevated in STP and DPS. An increase in these extreme events will clearly increase sediment loads. There will also be increasing concentration of sediment phosphorus if stream banks continue to erode into actively managed agricultural fields. Plain Language Summary Extreme precipitation events can cause streams to swell above their banks and erode near‐stream soil. Because stream bank soils contain phosphorus, increased erosion can increase the transport of phosphorus into lakes, leading to harmful algal blooms. Our study occurred in central Vermont along the Mad River, which flows into Lake Champlain. Tropical Storm Irene delivered a record amount of rainfall to much of Vermont. Soils were already wet and streams quickly flooded, causing drastic stream channel changes. We used aerial photography to measure the change in stream channel and calculated the amount of soil lost. We also sampled soil at six erosion sites, measuring various forms of phosphorus. These measurements suggest that one extreme event delivered as much sediment and phosphorus into the stre