Streambank alluvial unit contributions to suspended sediment and total phosphorus loads, Walnut Creek, Iowa, USA

Authors: BECK, WILLIAM - Iowa State University; ISENHART, THOMAS - Iowa State University; MOORE, PETER - Iowa State University; SCHILLING, KEITH - University Of Iowa; SCHULTZ, RICHARD - Iowa State University; Tomer, Mark

Submitted to: Water
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/25/2018
Publication Date: 1/28/2018
Citation: Beck, W.J., Isenhart, T.M., Moore, P.L., Schilling, K.E., Schultz, R.C., Tomer, M.D. 2018. Streambank alluvial unit contributions to suspended sediment and total phosphorus loads, Walnut Creek, Iowa, USA. Water. 10:111-133. https://doi.org/10.3390/w10020111.
DOI: https://doi.org/10.3390/w10020111

Interpretive Summary: Streambank erosion may represent a significant source of sediment and phosphorus lost from agricultural watersheds, but is rarely documented. Also, streambanks are often layered deposits and different layers may affect phosphorus transport and storage differently along the channel once eroded. This study measured sediment and total phosphorus (TP) losses from layered streambank sediments from a small (13,000 acre) watershed in Iowa during two years. Streambanks receded an average 7.3 inches with some differences among major deposits identified, as lower materials (i.e., bank toe) had the greatest recession. The total losses were as much as 44% of historic sediment loads, and represented nearly one ton of phosphorus (1940 lb) lost along the whole stream channel. This information is important to conservationists and watershed managers who need to understand the importance of streambank erosion and its contribution to water quality issues in agricultural watersheds in the Midwest.

Technical Abstract: Streambank erosion may represent a significant source of sediment and P to overall watershed loads, however, watershed-scale quantification of contributions are rare. In addition, streambanks are often comprised of highly-variable stratigraphic source materials (e.g., alluvial deposits), which may differentially impact in-channel P-dynamics once eroded. The objective of this study was to quantify sediment and TP losses from four materials comprising streambanks within a 5218 ha watershed in Iowa, USA. Streambank-face surveys, erosion pins, and soil analyses were used to quantify surface area representation, recession, and losses of sediment and total phosphorus (TP) over a two year period. Cumulative, whole-bank gross mean recession totaled 18.6 cm over two years, and material-specific gross mean recession ranged from 15.5 to 64.1 cm. Cumulative, whole-bank mean gross mass losses totaled 0.28 Mg sediment and 0.7x10-5 Mg TP per meter channel length. Annual sediment losses equated to 4-44% of historic suspended sediment loads. Stratigraphy was significant in gross material erosion and losses, with lower materials (i.e., bank toe region) exhibiting the greatest recession rates and cumulative recession. Weathered/colluvial material dominated total bank face surface area (88.3%), and contributed the greatest proportion of sediment and TP mass loss (66, 68%, respectively) versus other streambank materials.