NEW GEOLOGIC MAPPING ALONG THE MISSOURI NATIONAL RECREATIONAL RIVER

In cooperation with the National Park Service, South Dakota Geological Survey, Nebraska Conservation and Survey Division, and university partners, the U.S. Geological Survey began a geologic mapping project of the Missouri River corridor region along the Missouri National Recreational River (MNRR) of southeast South Dakota and northeast Nebraska. The geologic framework of the river corridor, especially the glacial and postglacial geology emphasized in our work, dominates the geomorphic and hydrogeologic bases of the physical habitat and ecology. The area of the corridor spanned by the MNRR includes several geologically significant terrains. The river valley, with a width of 3-6 km throughout most of the Dakotas and eastern Montana, expands markedly below Yankton, SD to a width of 10-16 km. The expanded reach coincides with the southernmost extent of the James River lobe of the late Wisconsinan Laurentide Ice sheet. In the uplands south of the river valley in eastern Nebraska, a much older and morphologically distinctive pre-Illinoian glacial terrain is partially mantled by loess. Both terrains include extensive glacial-buried-valley aquifers, which are contiguous with the alluvial-outwash aquifer that underlies the river and its valley.

The geomorphology of the river valley is dominated by post-glacial Holocene deposits and by the presently active fluvial system. Before the dam system was built during the mid 1900s, the river episodically transported large but variable fluxes of sediment from the extensive drylands upstream of the study area. Particularly in the expanded part of the valley, numerous crosscutting packages of meander-scroll morphology provide a record of changing fluvial processes that reflect response of basin hydrology and sediment yield that we are testing with mapping and chronology studies . Exposures enhanced by incision of the sediment-poor post-dam river commonly show well-sorted sand of point bars and channels overlain by overbank sediments and well-bedded channel fills dominated by silt, clay, and mud. In contrast, sparse exposures of glacial outwash have much coarser gravel dominated by crystalline lithology. At riverbank exposures of outwash, we observed an unusual abundance of native mussels that suggest a potential control on local water chemistry and ecology by these gravels through enhanced groundwater flux to the river.