LATE QUATERNARY INCISION AND TERRACE FORMATION IN THE LOWER CHIPPEWA RIVER VALLEY, WEST-CENTRAL WISCONSIN

The Chippewa River is a major tributary and source of bedload sediment to the upper Mississippi River (UMR) in west-central Wisconsin. During the Late Wisconsin, the lower Chippewa River (LCR) drained the Chippewa Lobe of the Laurentide Ice Sheet, which, at its maximum extent, covered over half of the Chippewa watershed. Overloaded with glacial outwash, the LCR aggraded and filled its valley with coarse alluvium. Then, as the glacial period waned, it incised, creating the Wissota Terrace, which today stands ~35 m above the river. The Wissota is not, however, the only terrace in the LCR valley. Rather, the valley contains remnants of six lower terraces, suggesting a Late Quaternary history marked by episodes of floodplain formation and incision.

The goal of our research is to reconstruct the chronology and causes of Late Quaternary incision and terrace formation in the LCR valley. To date, we have investigated the Wissota and three lower terraces along a transect ~60 km upstream from the Chippewa-Mississippi confluence, using optically stimulated luminescence (OSL) to determine the age of alluvial deposits and infer the timing of incision. Seven samples of Wissota alluvium, from depths of 0.9 to 4.3 m, yielded OSL ages between 9 and 10 ka. Four additional near-surface samples of alluvium from three lower terraces yielded OSL ages of 7.5 ka (T5), 4.7 ka (T3), and 2.3 ka (T1). Based on these ages, the LCR incised below the level of maximum Late Wisconsin aggradation in the early Holocene, several thousand years after the UMR incised, and continued incising at fairly regular intervals throughout the Holocene, a time when the UMR was slowly aggrading.

Our data clearly indicate that the effects of UMR incision and aggradation on the LCR were greatly diminished by other factors affecting river processes. One possibility of other factors include autogenic variations in sediment supply. E.g., as incision progressed up the LCR from the Mississippi, stored Late Quaternary sediment would have been remobilized from its valley and the valleys of rejuvenated tributaries, leading to periodic increases in the LCR's sediment load, possibly arresting the upstream advance of incision until the supply of remobilized sediment declined. Further research is needed, however, to test the plausibility of this—or any other—hypothesis.