| Summary: | The lower Mississippi valley (LMV) is a broad alluvial valley that contains large braided channel belts. Previous investigations have produced conflicting chronologies of channel-belt formation, primarily due to the lack of organic material for radiocarbon dating. Application of the optically-stimulated luminescence technique has produced a new chronology of LMV channel-belt formation and insight into LMV evolution during the last glacial cycle. Fluvial deposits range in age from last interglacial meander belts (85–83 ka) to multiple braided channel belts (64–11 ka) and record large-amplitude responses of the Mississippi River to allogenic forces. Slackwater deposits from the middle Mississippi valley and northern LMV indicate that the river was flowing at a position 18–30 m below present during the last interglacial and rapidly aggraded 15 m above the modern floodplain and switched to a braided regime by 64–50 ka, coincident with initial glaciation of the upper drainage basin. The Mississippi remained braided during glacial times until final meltwater withdrawal from its headwaters ca. 11 ka. Channel-belt formation and abandonment during deglaciation was controlled by high-amplitude fluctuations in meltwater and sediment discharge caused by the diversion of meltwater between the Mississippi River and North Atlantic. Correlation of channel-belt ages with calculated meltwater discharge suggests that channel belts formed during periods of reduced discharge, when meltwater from the southern margin of the Laurentide ice sheet was routed to the North Atlantic. During periods of high discharge, channel-belt surfaces were abandoned, and the river incised to lower levels. In the southern LMV, glacio-eustatic sea level influenced the elevation to which the channel belts were graded, causing late-glacial channel belts to dip below the Holocene floodplain. River-channel avulsions during the last glaciation have pinned the Mississippi River over regions of shallow bedrock in its present course down the ancient Iowa River ...
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