Authors: Douglas Faulkner*, University of Wisconsin-Eau Claire, Phillip Larson, Minnesota State University, Mankato, Hannah Adams, Missouri State University, Kira Kuehl, Minnesota State University, Mankato, Zach Hilgendorf, Arizona State University, Harry Jol, University of Wisconsin-Eau Claire, Garry Running, University of Wisconsin-Eau Claire
Topics: Geomorphology, Physical Geography, Water Resources and Hydrology
Keywords: glacial meltwater stream, base-level fall, transient river response, disequilibrium, Wisconsin
Session Type: Paper
Start / End Time: 3:05 PM / 4:45 PM
Room: Hoover, Marriott, Mezzanine Level
Presentation File: No File Uploaded
The Chippewa River, a moderate-sized tributary to the upper Mississippi River (UMR) in western Wisconsin, is a prime example of a paraglacial fluvial system in long-term disequilibrium. This state of disequilibrium began when the Laurentide Ice Sheet advanced into the Chippewa watershed ~30 ka, eventually covering 2/3 of the watershed at its maximum extent at ~21 ka. Glacial ice was gone from the watershed by ~13 ka, ending the river’s history as a meltwater stream. It left, however, a legacy of thick outwash fills in the valleys of the lower Chippewa River (LCR) and its major meltwater tributaries, along with periglacial fills of colluvium and alluvium in its non-meltwater tributaries. The LCR and its tributaries began removing those fills at ca. 18-16 ka and 13.5 ka, as knickzones migrated upstream in response to UMR incision during deglaciation. Terrace mapping, OSL dating of terrace fill, and GPR investigations of fill stratigraphy indicate that knickpoint migration and incision was prolonged and episodic, and largely controlled by autogenic fluctuations in sediment load derived from glacigenic valley fills. The LCR today is still a transient river responding to the effects of Late Wisconsinan glaciation and UMR incision. This is indicated by USGS sediment discharge data that reveal a 6-fold increase in specific sediment yield over a 40-km reach of the middle LCR, corresponding spatially to pronounced downstream variations in floodplain morphology, channel planform, channel stability, long profile, bar size, and bar armor. The modern river makes sense only in light of its glacial past.