DIFFERENTIATING MISSOURI AND JAMES RIVER SEDIMENTS IN SOUTH DAKOTA USING GRAIN SIZE AND GEOCHEMICAL METHODS

At the confluence of the James and Missouri rivers near Yankton, SD, mapping has revealed that historic channels of the James occupy former channels of the Missouri. Although the sources of sediments from these two rivers are different, identification of James versus Missouri river sands in superimposed channels has proven difficult. In order to accurately identify James and Missouri river sands, grain size and geochemical analyses of the sediments were performed. Samples were taken by hand auger from historic channels in the Meckling, Mission Hill, Jamesville, and Menno quadrangles in South Dakota, and Menominee and St. Helena quadrangles in Nebraska. In addition, sediments were collected from the modern Missouri and James river channels for comparison. Grain size analysis revealed few differences between Missouri and James River sediments. Missouri River sand is composed mostly of quartz and feldspar, reflecting upstream bedrock sources. James River sand is also composed predominantly of quartz and feldspar, but also contains abundant mud aggregates, which are consistent with a glacial till source area. Geochemical analysis of sand reveals that these river sediments have distinct major element concentrations reflecting different source areas and provides the most diagnostic criteria for sediment differentiation. Geochemical fingerprinting has allowed the identification of a historic James River channel that may correlate to a channel mapped by Lewis and Clark in 1804.