COMPARISON OF SEDIMENT CONTRIBUTION AND CHEMICAL CHARACTERISTICS OF THE FLOODPLAIN AT THE CONFLUENCE OF THE MISSISSIPPI AND MISSOURI RIVERS

The shared floodplain of the Mississippi and Missouri rivers in the southern half of the Elsah 7 ½ minute quadrangle map, eastern Missouri, is near the confluence of three major rivers: the Illinois River, which was an earlier Mississippi River, the present Mississippi River, and the Missouri River. The Mississippi and Missouri rivers have distinct characteristics, which are reflected in different styles of deposition, fluvial architecture, and thicknesses of deposits on this shared floodplain. The deposits of the two rivers were distinguished using mineralogy, depositional differences, grain-size analysis, and chemical analysis of the sediments. The sediment contribution to the floodplain reflects the relative amount of suspended sediment carried by each river. Prior to settlement by European colonists, the Missouri River is estimated to have contributed twenty times the suspended sediment load as that of the upper Mississippi River. Presently, the absolute amount of suspended sediment in the Missouri River has decreased to approximately ten times that of the upper Mississippi due to river management. This change is reflected in the deposits on the floodplain.