Historical Floodplain Sedimentation in the Galena River Basin, Wisconsin and Illinois

Abstract

Abstract Post-settlement alteration of the natural land cover accelerated floodplain sedimentation in the Galena River basin in Wisconsin and Illinois. Twenty-three stream channels and valley cross sections originally surveyed by the Soil Conservation Service c. 1940 were resurveyed in 1979. A comparison of the two surveys shows that bankfull cross-sectional area has generally decreased since 1940, especially in the upper and middle portions of the basin where significant reduction in bankfull stream width occurred. The magnitude of post-1940 floodplain sedimentation increased in the downstream direction, but variations in magnitudes are associated with both valley width and drainage area changes. Valley width influences on hydraulics largely govern the pattern of overbank sedimentation. An analysis of all historical floodplain sedimentation showed that abnormally wide valleys were associated with large accumulations despite their having low values for mean depth. Abnormally narrow valleys had minor accumulations and low mean depths of historical sediments, apparently because narrow valleys tend to promote sediment transportation during floods. During large floods, zones of deposition are immediately upstream or downstream of a valley constriction, but sediment transport or erosion is common in narrow valleys. Average rates of post-1940 sedimentation in the lower portion of the trunk stream are 0.75 cm/yr compared to 1.9 cm/yr for the pre-1940 historical period. Adjustments of the stream channels to a less variable flow regime and the modest rates of overbank sedimentation primarily reflect soil conservation practices implemented in the 1940s and 1950s. These observations support the concept that stream channel and floodplain responses to hydrologic changes occur rapidly in humid regions.