Mapping Post-Glacial Terraces in the Lower Cannon River Valley, southeastern Minnesota: Geomorphic and Archaeological Significance

Authors: Adria Slade*, University of Wisconsin - Eau Claire, Nicole Marcou, University of Wisconsin-Eau Claire, Garry Running, University of Wisconsin-Eau Claire, Douglas Faulkner, University of Wisconsin-Eau Claire
Topics: Geomorphology
Keywords: Fluvial terraces, Geoarchaeology, Midwest-USA
Session Type: Poster
Day: 4/5/2019
Start / End Time: 8:00 AM / 9:40 AM
Room: Lincoln 2, Marriott, Exhibition Level
Presentation File: No File Uploaded

The Lower Cannon River (LCR), an important but poorly understood archaeological area in southeast Minnesota, was a glacial meltwater tributary of the Upper Mississippi River (UMR) during the Late Wisconsinan. During deglaciation, the LCR responded to deep UMR incision by incising into its valley’s outwash fill (based on studies of other UMR tributaries). Today, remnants of numerous terraces document the LCR’s incision history. This study’s objective was to lay a foundation for unraveling that history by ascertaining the heights and spatial patterns of terraces within the LCR valley. Using LiDAR-derived DEMs in ArcMap, we manually identified terrace remnants, mapped their treads, and calculated their heights (above the modern river). Since heights were found to range continuously (from 4 to >40 m), we grouped them into equal-interval height classes (4 m). We then divided the valley into six 5-km reaches and determined the areal extent of each height class in each reach. We discovered terraces are most common (based on areal extent) in the lowermost and uppermost reaches, where the bedrock valley is widest, with the lowermost reach dominated by higher terraces and the uppermost by lower ones. While relatively scarce, remnants in the narrow middle reaches suggest a transition from higher to lower terraces in the upstream direction. Based on these findings, we hypothesize that LCR incision was nearly continuous as it propagated upstream from the UMR, with little time for floodplain formation. Therefore, cultural material of archaeological interest is likely accessible to standard shovel testing methods used by archaeologists.

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