GEOLOGIC MAPPING AND RESEARCH ALONG THE MISSOURI NATIONAL RECREATIONAL RIVER, NEBRASKA AND SOUTH DAKOTA: INTEGRATION OF RESULTS AND SIGNIFICANCE TO HYDROGEOLOGIC AND ECOLOGIC ISSUES

Ongoing geologic research, including new geologic mapping, geochronology, and geochemistry, has focused on the river corridor area of the reach of the Missouri National Recreational River (MNRR) downstream from Gavins Point Dam. This work is supported by the USGS National Cooperative Geologic Mapping Program (FEDMAP, STATEMAP, EDMAP), the National Science Foundation, the National Park Service, two State Geological Surveys, and three university geology/river studies departments. Central to this work is geologic mapping of the floodplain to delineate and test depositional facies models, and to map past positions of a very dynamic river system in postglacial time with scores of 4-7 m deep auger holes and age control from optically stimulated luminescence dating. Postglacial deposits of the valley floor are bounded on the north and underlain by late Wisconsin glacial and glaciofluvial deposts of the James River lobe of the Laurentide ice sheet that advanced to this part of the MNRR area across a partly forested parkland that has yielded many ¹⁴C ages on wood of ~12,500 ¹⁴C years BP. The re-advance was followed closely by calcite precipitation from groundwater, as constrained by mass spectrometric U-series, Sr, and stable isotope data. New mapping results from the Wynot 7.5-minute quadrangle in Nebraska contrast markedly to the James lobe terrain in South Dakota; the mapping shows that uplands south of the MNRR include a discontinuous to thick loess mantle over a complex of pre-Illinoian till, gravel, sand, and silt with little spatial correspondence to the younger superimposed southeast-oriented valley network. There is particular interest in understanding the relation of buried Pleistocene glaciofluvial gravel to the modern river channel; these gravel deposits may be a primary control on groundwater/surface water interactions as well as a source of gravel-cobble substrate needed for spawning of endangered sturgeon. Our geologic work is designed to provide data for adaptive management of this segment of the Missouri River, including information to help guide restoration of habitats for native fishes and shorebirds. A NPS Geologic Resources Division scoping meeting in August 2008 facilitated the communication and discussion of these results and further honed the orientation and needs for geologic mapping.