LATE QUATERNARY TERRACE AND LONG PROFILE DEVELOPMENT IN THE LOWER CHIPPEWA VALLEY

Recent mapping and fieldwork in the Upper Mississippi and Chippewa River valleys provide an opportunity to integrate new data with previous work, shedding light on the complex behavior of fluvial systems in the midcontinent of North America. Through much of the last glacial period, streams in the Upper Mississippi River Valley were dominated by aggradation within their valleys. Incision into this late Pleistocene floodplain produced a series of broadly correlative terrace treads. Along the Mississippi River, this tread is referred to as the Savanna Terrace. Where this tread is exposed in the Lower Chippewa River Valley, a major tributary of the Mississippi River, it is referred to as the Wissota Terrace. Recent mapping and fieldwork in the Lower Chippewa Valley provides an opportunity to compare the fluvial evolution of the Mississippi River to several of its tributaries and illustrate the complex behavior of post-glacial fluvial systems. The Savanna and Wissota Terrace treads have steeper gradients compared to the active beds of the Mississippi and Lower Chippewa Rivers. However, in many tributaries of the lower Chippewa River, the Wissota Terrace gradient is shallower than the stream gradient. The current stream profiles of these lower-order streams diverge from that of the Wissota Terrace tread. This pattern may reflect a difference in the underlying controls of sediment storage and transport within a drainage basin. Large trunk streams, such as the Mississippi River appear to be responding to climatic factors related to sediment and water supply, while smaller streams are responding to the resulting change in base level of the trunk streams they flow into. Radiocarbon data obtained from charcoal and terrestrial snail shells, combined with a review of previously published dates suggests that formation of the Savanna Terrace predates well-known outburst flood events from glacial Lake Agassiz. Instead, this incision may relate to the widespread ice instability during the Heinrich H1 event. By comparison, formation of the Wissota terrace occurred well after the Mississippi River incised its floodplain.