Fluvial System Response to Abrupt Base Level Fall: Mapping Tributary Stream Terraces in the Lower Chippewa River Watershed

Abstract

During the Late Wisconsinan, the Laurentide Ice Sheet (LIS) advanced from Canada towards the Chippewa River Watershed. Approximately 30,000 years ago (30 ka), the Chippewa Lobe of the LIS entered the watershed and reached its maximum extent by around 25 ka. Approximately 20 ka, the ice began to retreat and had pulled out of the watershed by 13 ka. During this time, the LCR and its two largest tributaries, the Red Cedar River, and the Eau Claire River, were glacial meltwater streams, overloaded with glacial outwash that filled their valleys. It has been estimated that the glacial ice was completely retreated from the LCR around 13 ka. Past research by Faulkner and others aimed at reconstructing how the Chippewa River responded to base-level fall, which occurred when the Upper Mississippi River (UMR) incised during deglaciation of the Upper Midwest. The UMR is the base level for the Chippewa River and acts as the control for how far it can incise. When base level drops, it can trigger incision, or active downcutting, of the river. Faulkner et al. (2016) proposed a model of episodic knickpoint migration and incision up the LCR in response to UMR incision. The objective of our study is to test the model by focusing on the incision of tributaries to the LCR, building on the research of Hilgendorf and Faulkner (2015). Our hypothesis is that tributaries that join the LCR farther upstream from the UMR would have more terraces than those downstream.