Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: 6/3/1999
Publication Date: N/A
Interpretive Summary: The loess area of the midwestern United States contains thousands of miles of unstable stream channels that are undergoing systemwide channel-adjustment processes as a result of (1) modifications to drainage basins dating back to the turn of the 20th century; 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, near the turn of the 21st century, 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 loess and a 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 loess 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.
Technical Abstract: The loess area of the midwestern United States contains thousands of miles of unstable streams, responding to human modifications imposed near the turn of the 20th century. The dominant process is channel widening and meander extension by mass failure. The most severe, widespread instabilities are in western Iowa where a thick loess cap and a limited supply of coarse-grained (sand and gravel) material restricts bed-level recovery by aggradation. High streambanks combine with low cohesive strengths to sustain channel widening. Channel adjustments in West Tennessee and southeastern Nebraska are not as severe as in western Iowa. Channels in west-central Illinois and east-central Iowa appear to be closer to recovery than in other parts of the region. Degradation rates for streams draining the loess hills of western Iowa and eastern Nebraska have decreased nonlinearly since 1920, approaching minimal values. In some downstream reaches, sandy alternate bars and fluvially deposited sand on low-bank surfaces indicate bed-level recovery and the "aggradation" stage of channel evolution. This "natural" recovery process which often occurs in other unstable stream systems following 10-15 years of incision, was delayed for 55 years along some reaches because of the lack of hydraulically-controlled (sand- or gravel-sized) sediments. Silts from eroding beds and banks were easily transported through tributary systems to the Missouri River. With loess thickness generally decreasing with distance upstream and with the degradation process migrating upstream with time, incision ultimately resulted in the exposure of coarser-grained glacial till, a source of coarse sediment for downstream aggradation and bed-level recovery.