DRIFTLESS AREA CHANNEL AND SEDIMENT RESPONSES TO LATE 20TH-EARLY 21ST CENTURY CHANGES IN LAND COVER, CLIMATE, AND RUNOFF HYDROLOGY

Separating the effects of land conservation practices from other natural environmental factors that influence channel and fluvial sediment dynamics has been confounded by limited long-term quantitative observations. Here we address this problem through a 2009-2010 re-survey of southwest Wisconsin Driftless Area channel cross sections previously surveyed in 1982. The resurvey shows a general tendency toward net erosion (average about 5 percent increase in cross-section area) when data from all channels are combined. Taking into account drainage areas, channels draining less than 50 km² experienced greater net erosion (average about 11 percent size increase in cross-section area) than the general tendency and channels draining between 50 and 250 km² experienced net deposition (average about 9 percent decrease in cross-section area). Channel adjustments have been largely focused on the banks because channel beds commonly are armored with cobble-boulder gravel lag of chert. The general tendency for net erosion since 1982 is unexpected because Driftless Area flood magnitudes of all return frequencies have generally decreased since then compared to the period c. 1940-1981. The reduction in flood magnitudes results mainly from recent (post c. 1950) improved land conservation related to increased plant density for row crop corn agriculture, greater use of low- or no-till planting techniques, and decreased grazing coupled with forest expansion on steep hillslopes. These three factors have significantly reduced upland sediment delivery to streams where, in turn, the relatively lowered sediment concentrations associated with upland runoff favors net erosion of channel margins. Furthermore, flows exceeding the normal bankfull stage of ~1.5 years recurrence frequency tend to be higher-energy than would be expected because most are confined between banks whose heights have been increased by historical sedimentation. During the late-19^th and early 20^th centuries, when land conservation practices were generally lacking, sediment concentrations were high and flood flows spread across much of the valley floor which then was part of the active floodplain. The re-survey results show that sediment mobility and storage sensitivities to environmental change vary systematically within the drainage hierarchy.