Hungry Canyons Alliance: Streambed Stabilization in Western Iowa

Abstract

The Hungry Canyons Alliance (HCA) was formed locally to provide financial and technical assistance for streambed stabilization projects in the 19 counties of the deep loess region in western Iowa. Stream straightening and land-use changes, coupled with the erosive nature of loess, caused streams to downcut and widen, undermining bridges, farmland, and utility lines. The most efficient and affordable way to stop channel degradation has proven to be the construction of grade control structures (GCS). GCS allow a drop in stream elevation in a controlled setting, prevent further degradation, decrease sediment loads and turbidity, and increase water quality. With over 750 GCS of all types in western Iowa, western Iowa has been referred to as a “laboratory” for GCS design. The HCA has conducted research on GCS design and fish passage, the use of directional drilling to control gully erosion, the use of scrap tires in GCS, knickpoint migration, and aerial stream classification. THE PROBLEM In western Iowa, 19 counties contain deposits of loess ranging from 4 m to over 61 m deep. Loess is highly erodible and very susceptible to stream channel erosion and degradation. Beginning with settlement of the area around the late 1840’s and early 1850’s, land use changed dramatically from prairie to farmland, likely causing rapid erosion in upland areas and deposition in stream networks. Stream channel dredging and straightening from the early 1900’s to the 1950’s was done to reduce flooding, increase the amount of farmland in production, and to clean out the sediment clogged channels. After straightening, streambed bed slopes were steeper because streams still had the same fall per mile, yet there was a shorter distance over which that fall occurred. The steeper bed slopes increased water velocities, increasing stream erosion. Streams could either re-meander or downcut. Due to the high erodibility of loess soils, streams downcut, causing accelerated soil erosion in western Iowa (Fig. 1). Knickpoints, which are naturally occurring overfalls (Fig. 2), would form and rapidly advance upstream, eventually affecting the entire watershed. Unstable, vertical stream banks would slump to a more stable slope; if high flow events flushed the slumped bank debris, vertical banks would again be unstable and slump, widening the channel. Tributaries of degraded streams would also erode and adjust to the lowered base level, often initiating gully formation where no channel had previously existed.