WHAT′S MISSING? WHAT′S LEFT BEHIND? HOW HUMAN ALTERATIONS TO LAND USE AND DRAINAGE ARE RECORDED IN DIFFERENT FLUVIAL ENVIRONMENTS IN AN INCISING AGRICULTURAL LANDSCAPE OF THE UPPER MIDWEST, USA (Invited Presentation)

Traditional depositional archives (lakes) provide invaluable records of changes in sedimentation associated with human alterations to landscapes but limited detail on the response of specific upstream geomorphic environments. In the Le Sueur River in Minnesota, USA, we are utlizing records on terraces to determine the effect of land use change on different geomorphic environments. The Le Sueur is incising through glacial sediments, leading to low-gradient uplands with deeply-incised river valleys. Downstream lake cores indicate a ten-fold increase in sedimentation starting in the mid-1800s, with most sediment derived from upland fields followed by a shift in the mid-1900s from field to streambank/bluff-sourced sediments, indicative of increasing discharges and more erosive rivers. Do we see the same signal in the erosional basin upstream?

As the river incises and meanders, terraces are created that provide one of the few sediment archives in the erosional half of the basin. Ravines connecting upland fields to incised channels can be disconnected through meander migration. When this happens, ravines build alluvial fans, which record the effects of land use change locally. We investigated 5 ravine fans by coring to obtain sediment samples and using fly ash as a marker of Western settlement. Minimum fan ages were set to the age of the terraces they overlie. Results show that all fans experienced an increase in deposition at the time of settlement. In most ravines, this was followed by incision, indicating an increase in transport capacity associated with changes in upland drainage.

On mainstem channels, increasing discharge has led to channel widening as seen in historic photos that date back to the 1930s, but to see the full effect of hydrologic change on channel geometry, we again utilize records on terraces. We imaged and mapped a series of five meander cut-offs on terraces using GPR and hand augering. Channel geometry and bankfull discharge from paleochannels were compared to the modern channel and gage records over the past 70 years. Results show that channels have widened and deepened, with bankfull discharge increasing 2 to 20x over pre-settlement values. This aligns well with a shift in sediment sources from upland to near-channel and with a shift in ravine fan behavior from depositional to erosional.