Channel Morphology Changes and Sediment Yield for a Small Agricultural Watershed in Iowa

Abstract

ABSTRACT CHANNEL widths, depths, and cross-sectional areas were measured at several locations on a stream draining a 50.5-km2 agricultural watershed in east-central Iowa several times during the 1964 to 1980 period. Land use became more row-crop intensive during this period, with the channel progressively increasing in width and depth. The average annual increase in channel area from 1964 to 1980 was 2.7% (0.29 mVyr), with the larger increases in channel dimensions associated with the 1977 to 1980 period. Runoff and sediment loads from the watershed, and field sub watersheds, were investigated for 1976 to 1980. At the field sub watersheds, snowmelt runoff was a sizeable portion of total annual runoff, yet erosion and sediment transport with snowmelt was minimal. For the stream, snowmelt sediment load (as a percentage of the total annual load) was larger than at the field sites, suggesting the possible reentrainment by snowmelt of previously deposited sediment. However, cross-sectional data for 1964 to 1967 did not substantiate the hypothesis of sediment deposition in the stream channel during the summer, with subsequent flushing of deposited sediment during snowmelt. Average sediment load attributable to channel contributions was estimated to be at least 25%.