FLUVIAL HISTORY OF THE LOWER WABASH VALLEY: SLACKWATER, TORRENTS, LAZY RIVERS

The lower Wabash River valley was a major meltwater outlet throughout the Pleistocene, and today comprises an actively meandering alluvial system, tributary to the Ohio River. Bedrock undergirds the surrounding uplands. Within the valley the bedrock surface relief is high. It is both buried by up to 45 m of fluvial sediment, but also occurs as isolated knobs or is exposed in the modern river bed. Ice advanced to within 10 km of the river mouth during the Illinois Episode, but only reached 70 km upstream during the Wisconsin Episode. During each glacial episode, outwash filled the valley to form slackwater lakes in tributary valleys. At the mouths of these slackwater lakes, outwash prograded upgradient during high flows. Illinois Episode deposits were mostly eroded out of the valley fill, but dating of Wisconsin Episode deposits by OSL methods constrains most slackwater lake and glacifluvial deposits to 45-22 ka. One or more jökulhlaup at the end of the Wisconsin, possibly sourced by different glacial lobes, are evident in the lower valley mainly by erosional landforms; large caliber sediment typical of correlative deposits near the terminal moraines are largely absent. Where floodwaters overtopped valley walls, straths with channel sands and levee deposits were formed. In one location, bedrock valley walls apparently steered flood flows to carve a remarkably straight and long scarp about 10 km long and 5-10 m high into a (now) terrace of slackwater lake sediment. The relative elevation of this terrace and a lower terrace interpreted as the channel floor suggests that the flood flows were ~10 m deep. The post-jökulhlaup braid plain was apparently active until ~17 ka, after which time the river transformed into a meandering system. Inset channel belts exhibit overlapping spatial relationships. Older, high sinuosity, meander belts migrated progressively eastward, but the current, less sinuous, meander belt has been migrating largely eastward, a trend previous authors attributed to tectonic tilting.