Sediment and phosphorus contributions from eroding banks in a large intensively managed watershed in western Iowa, United States

Authors: WILLIAMS, FORREST - Iowa State University; MOORE, PETER - Iowa State University; ALLEN, JADE - Iowa State University; ISENHART, THOMAS - Iowa State University; THOMAS, JOHN - Hungry Canyons Alliance (HCA); Kovar, John; SCHILLING, KEITH - Iowa Geological Survey

Submitted to: Journal of the American Water Resources Association
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/12/2023
Publication Date: 9/21/2023
Citation: Williams, F.F., Moore, P.L., Allen, J.V., Isenhart, T.M., Thomas, J.T., Kovar, J.L., Schilling, K.E. 2023. Sediment and phosphorus contributions from eroding banks in a large intensively managed watershed in western Iowa, United States. Journal of the American Water Resources Association. 60(1):148-162. https://doi.org/10.1111/1752-1688.13164.
DOI: https://doi.org/10.1111/1752-1688.13164

Interpretive Summary: Losses of phosphorus (P) from agricultural lands represent a major cause of water quality impairment within Iowa and elsewhere. Producers and land managers have made progress over the last several decades in reducing soil-bound P losses from agricultural lands to receiving waters via soil erosion. However, there is a growing body of evidence that much of the sediment and P delivered to the surface waters from agricultural landscapes originates from stream banks. Working in the Nishnabotna River watershed in southwestern Iowa, we used a multi-step process that included a new remote sensing tool used along with physical sampling of river bank sediments to first map channel migration patterns and then to estimate the net contribution of bank erosion to sediment and P losses. During the period between 2009 and 2018, we found that roughly 43% of the P that entered the Nishnabotna River resulted from bank erosion due to river channel migration. We also found that the larger streams and tributaries in the watershed contributed relatively more to sediment and P losses, even though the total length of these tributaries is much smaller. Future attempts to decrease losses of sediment and P should focus on controlling bank erosion in the larger streams and tributaries in the watershed. Despite limitations in our analysis, an improved understanding of streambank P contributions will help natural resource managers make improved recommendations for effective soil and water conservation practices that best reduce P loading to rivers.

Technical Abstract: In this study, a new remote sensing tool was used in conjunction with sampling of river bank sediments to map channel migration patterns and estimate the net contribution of bank erosion to the sediment and phosphorus (P) budget of the Nishnabotna River in southwest Iowa. Between the years of 2009 and 2018, we found that at least 1.81 ± 0.57 x 10^7 Mg of sediment and 8.26 ± 2.5 x 10^3 Mg of P entered the Nishnabotna River due to channel migration. This equates to 0.38 Mg of sediment per meter of channel per year, 0.18 kg of P per meter of channel per year, and roughly 43% of the total watershed P export. Our results also indicate that the contribution of net volume loss by stream order increases sharply from third to sixth order, even though the total channel length is much smaller in the higher orders. These results suggest that bank erosion is an important source of sediment and P within the watershed and that future attempts to decrease riparian exports of sediment and P should focus on controlling bank erosion in high order reaches.