Knickpoint Migration and Evolution in the Deep Loess Region of Western Iowa

Knickpoints are geomorphic bed features characterized by the formation of a sharp change (i.e., knick) in the channel slope leading to the creation of a wide crest staircase. Knickpoints migrate upstream and cause bank instability, exposure of bridge-crossing foundations, and the deterioration of fish habitat. They are a common geomorphic hazard in many streams of the Midwestern United States that is partly due to prevailing management practices, including stream channelization, which mostly occurred during the first half of the 20th century. Over the last 40 years, local governmental agencies have attempted to halt knickpoint migration by constructing different types of grade control structures (GCS). Despite these attempts, the problem still persists. Understanding of the mechanisms triggering the onset of knickpoint migration is at a primitive stage. In this study, a field evaluation of a representative channel reach containing a knickpoint in the Deep Loess Region of western Iowa was used to test current theoretical and numerical models of knickpoint migration. Continuous stage and periodic flow measurements were recorded for almost two years and included an extreme event, which occurred during the catastrophic 2008 Midwestern United States floods. In addition, soil samples from the river bed and banks were collected and the bathymetry was measured to fully characterize the stream reach. The data were used to estimate the parameters of various models to quantify the knickpoint migration rate and evolution during various hydrological events. The final goal of this research is to identify the most suitable approach to model the knickpoints found in Midwestern streams. The results of the study can then be used by federal or state agencies to define better plans for the future control of bed degradation using more adequate GCS.