THE GEOMORPHOLOGY, ALLUVIAL DEPOSITS, AND NATURAL DIVERSIONS OF THE MIDDLE MISSISSIPPI RIVER VALLEY

A high-resolution DEM (± 0.2 m vertical accuracy), boring logs, and historic channel-bed data were used to investigate the geomorphology, alluvial fill, and natural diversions of the Middle Mississippi River Valley (MMRV). Analysis of the DEM revealed three different channel regimes and the absence of well developed terraces or tributary fans. The oldest channel regime is a braided system of which only isolated remnants remain. The braided system is cross-cut by a meandering channel regime. Low relief eolian landforms (e.g., transverse dunes) were observed cross-cutting some of the meanders. The current channel form of the MMR is an island braided regime.

The review of over 200 boring logs revealed depth to bedrock in the MMRV alluvial valley ranges form 5 to > 60 m below ground surface. Significant variation in depth to bedrock between closely spaced borings suggests a high-relief bedrock surface, perhaps of karstic origin. In the overlying alluvium, where the floodplain has not been reworked recently, there is a distinct assemblage of inter-bedded brownish, silt, loams and grey, silty, clay (generally >10 m thick) which overlies older, coarser alluvial deposits. This assemblage is interpreted as interbedded loess and overbank deposits. Near the base of this unit, red to reddish-brown silt and clay beds were found in about a dozen of the borings. The reddish color of these beds suggests a possible Lake Superior basin provenance.

Large-scale Pleistocene avulsion events are recorded at Thebes Gap and Grand Tower. The Thebes Gap diversion has been studied previously, but little is known about the diversion at Grand Tower (GT). The modern course of the MMR in the area of GT appears to be the result of at least two diversions. The first diversion routed the channel around Fountain Bluff and then back into the alluvial valley. The second diversion occurred just downstream of the original, resulting in the current position of the river along the Missouri bluffs. This new course is primarily bedrock-floored. In contrast, bedrock depth within the abandoned portion of the original diversion is > 50 m. The location of these diversions and varying depths to bedrock coincide with elements of the Ste. Genevieve Fault Zone, suggesting some structural control of the GT diversions.