Lateral Stream Migration Rates in the Blue River Watershed, Wisconsin

Abstract

Lateral channel migration is a naturally occurring process within meandering rivers when adjusting to a relatively stable state. Factors like stream power, soil, and streambank vegetation can influence the rate of migration along a channel. Understanding the processes controlling channel migration can be used to determine locations susceptible to nutrient and metal contaminant release and sources of suspended sediment. Many of the recent studies on bank erosion have worked at the small scale of individual sites or reaches, therefore, it is important to examine the spatial patterns and controlling processes at the watershed scale to gather new perspectives and determine the strength of relationships on a larger scale. The purpose of this study was to determine whether migration rates have varied temporally and spatially within the Blue River watershed of southwestern Wisconsin. Furthermore, the effect of changing land use and land management practices on bank erosion were determined using historic survey data from 1830 and aerial imagery from 1940, 1968, 1995, and 2010 with geographic information science (GIS). Potential controlling factors were also determined by statistically comparing stream power and exanimating spatial links between stream power and geomorphic data. Results showed that changes in land management practices have impacted the watershed and the individual reaches within the watershed, wherein there was a general decline of migration rates within two of the reaches as land management practices improved, whereas relatively constant rates were observed within the other reaches. The individual reaches mostly experienced low migration rates within the headwater streams where valleys were narrow and underlain with resistant dolomites, either increased or were consistent throughout mid-reach locations, and generally decreased farther downstream as valleys widened in exposed friable Cambrian sandstones. Stream power was a not a strong, singular explanatory variable, however, there were several reaches where high stream power was associated with high migration rates. Changes in lithology, slope, and sections of channel confinement explain some of the spatial variability in downstream trends. These results can be used in future studies estimating suspended sediment volumes and metal contaminant release from bank erosion within the Blue River watershed. This study also provides evidence of the importance of conducting studies at the watershed scale to better understand the spatial variability of geomorphic process and therefore potentially provide more organized and time-efficient approaches to watershed management.