CHANNEL INSTABILITY IN THE LOESS AREA OF THE MIDWESTERN UNITED STATES¹

Abstract

The bess area of the midwestern United States contains thousands of miles of unstable stream channels that are undergoing system-wide channel-adjustment processes as a result of (1) modifications to drainage basins dating back to the turn of the 20th century, including land clearing and poor soil-conservation practices, which caused the filling of stream channels, and consequently (2) direct, human modifications to stream channels such as dredging and straightening to improve drainage conditions and reduce the frequency of out-of-bank flows. Today, many of these channels are still highly unstable and threaten bridges, other structures, and land adjacent to the channels. The most severe, widespread instabilities are in western Iowa where a thick cap of bess and the lack of sand-and gravel-sized bed sediments in many channels hinders downstream aggradation, bed-level recovery and the consequent reduction of bank heights, and renewed bank stability. In contrast, streams draining west-central Illinois, east-central Iowa, and other areas, where the bess cap is relatively thin and there are ample supplies of sand-and gravel-sized material, are closer to recovery. Throughout the region, however, channel widening by mass-wasting processes is the dominant adjustment process.

Streambank erosion in two watersheds of the Central Claypan Region of Missouri, United States

“…In the Forked Deer River system in western Tennessee, Simon et al (1996) reported that streambank erosion accounted for up to 80% of the in-stream sediment. Recent work utilizing stable isotopes has shown that streambank erosion can account for the majority of suspended sediment present in streams during high flow conditions (Wilson et al 2008).…”

Bank‐collapse processes in a valley‐bottom gully, western Iowa

“…Mass failures are important because they rapidly move large volumes of colluvium to the bottom of a gully, where the colluvium becomes susceptible to erosion by fluvial action. The result can be unusually high sediment concentrations during runoff, with major impacts on siltation and nutrient loading in higher order streams (e.g., Piest et al, 1975;Sharpely et al, 1996;Simon et al, 1996). In 56 catchment studies reviewed by Poesen et al (2003), sediments derived from gully erosion accounted for 10-95 per cent of total sediment losses from the catchments.…”

An empirical–conceptual gully evolution model for channelled sea cliffs

“…At the most fundamental level incision is a requirement of drainage network evolution and development, but it can also rejuvenate fluvial networks and lead to further landscape development . As such, the sediment output from a network of incised channels will often represent much of the sediment yield from a landscape (Schumm et al, 1984;Simon et al, 1996). The derived sediment is stored in bars, floodplains and terraces and therefore has a significant impact on downstream morphology and ecology.…”