DEPOSITIONAL HISTORY OF A SHARED MISSISSIPPI/MISSOURI FLOODPLAIN USING GEOMORPHOLOGY, SEDIMENTOLOGY, AND CHEMISTRY

On a shared floodplain of the Mississippi and Missouri rivers near their confluence, the two channels come within 2.4 km of each other. However, it appears that the two rivers do not share the floodplain equally. Twenty deep cores and 17 shallow cores were collected in the study area. Geomorphology, sedimentology determined using core descriptions and grain-size analysis, and chemical analysis of the fine sand fraction show that deposition on this shared floodplain is dominated by the Missouri River. This indicates that the Missouri River channel is relatively more active, has migrated extensively across the floodplain, and has dominated floodplain sedimentation. In contrast, the Mississippi River channel is relatively stable and has been confined to a limited area along the northern margin of the floodplain. A probable cause for this inequality of channel occupation and sedimentation across the shared floodplain is that the Missouri River has a much higher suspended sediment load than the Mississippi River. We anticipate optically stimulated luminescence (OSL) dates of floodplain sediment will allow us to recreate the depositional history of this floodplain.