THE PARAGLACIAL LOWER CHIPPEWA RIVER, WEST-CENTRAL WISCONSIN, USA (Invited Presentation)

Former proglacial meltwater streams in the Upper Midwest of the U.S. provide excellent opportunities to investigate the transient response of inland alluvial streams to abrupt base-level fall. The lower Chippewa River (LCR), a moderate-sized tributary of the upper Mississippi River (UMR) in west-central Wisconsin, is one such river. The LCR lies beyond the Late Wisconsinan terminal moraine of the Laurentide Ice Sheet, which covered two-thirds of the Chippewa River watershed at its maximum extent (~22-21 ka). During the time glacial ice was in the watershed (~30-13 ka), thick fills of outwash accumulated in the valley of the LCR and its major meltwater tributaries, along with fills of periglacial colluvium and alluvium in its non-meltwater tributary valleys. During regional deglaciation, the LCR experienced at least two episodes of significant base-level fall due to UMR incision: 15 m at 18-16 ka and 40 m at 13.4 ka. In response, incision propagated up the LCR, leaving the highest level of Late Wisconsinan aggradation as the Wissota terrace. Diffusion modeling of LCR incision suggests the river should have incised and abandoned the Wissota terrace along its entire length (120 km) well before 10 ka. However, OSL ages on alluvium from stream terrace fill document a much slower process: incision had propagated up the LCR valley less than 55 km by 10 ka and likely did not reach the Late Wisconsinan terminal moraine until 5 ka or later. Furthermore, the height and spatial arrangement of terrace remnants and the stratigraphy of their alluvial fills indicate that incision has been an episodic process, while the morphology of the contemporary river and its floodplain suggests it continues to the present day. This prolonged and episodic process implicates autogenic fluctuations in sediment load—the result of stream incision and incised-channel evolution in glacigenic valley fills—as the primary control on the LCR’s transient response to UMR incision and abrupt base-level fall.