Sediment and Phosphorus are major sources of pollution and impairment within rivers and streams. Excess sediment input to rivers and streams can encourage the accelerated growth of harmful algal blooms, which in turn can be responsible for waterbody eutrophication, and the creation of coastal hypoxic zones. These issues are particularly relevant within Iowa, where land use changes, such as the conversion of land to row crop agriculture, installation of subsurface drainage, and straightening of waterways have led to increases in the stream-power of Iowa’s waterways. This in turn has resulted in increasing rates of bank erosion within Iowa’s river and streams. Although the amount of sediment and phosphorus contributed by eroding banks is still poorly constrained, researchers agree that bank erosion is an important part of riparian sediment and Phosphorus budgets. The 2013 Iowa Nutrient Reduction Strategy was developed to assess and reduce nutrients to Iowa waters and the Gulf of Mexico, but a lack of data led an inability to constrain the role of bank erosion within Iowa’s sediment and Phosphorus budgets. Methodologies for measuring bank erosion do exist, but few can reliably measure bank erosion within large watersheds. Herein we present the Aerial Imagery Migration Model (AIMM), a new tool that utilizes automated extraction of river channels from aerial imagery and a DEM analysis to track patterns of stream channel migration. AIMM’s reliability was assessed within the South Fork Iowa River Watershed, and then implemented, in conjunction with soil sampling, within the Nishnabotna watershed. This analysis predicted that within the Nishnabotna watershed, eroding bank inputs of Phosphorus are likely greater than 0.18 kg of P per year per meter of channel length. Additionally AIMM predicted that the majority of bank erosion occurs within high order reaches that are not typically considered in bank erosion studies. This finding has important management implications that could change the ways in which we address sediment and Phosphorus loss within Iowa’s watersheds.