MAPPING THE MISSOURI RIVER FLOODPLAIN SUBSURFACE: A HOLOCENE HISTORY OF CLIMATE CHANGE

Undergraduate students are preparing a first set of surficial geologic maps of the Missouri

River floodplain as part of a three-year program funded by NSF-REU and the USGS EDMAP Program. Mapping surficial strata of the Missouri River Floodplain gives insight into the mechanics of meandering river systems and the rates at which the river evolves due to climate change for one of America's largest rivers.

The data collected during the summer of 2009 served to complete a 40-km stretch of the river valley between Sioux City, IA and Decatur, Ne. This data has unearthed many questions regarding the evolution of a major river system, including bed rock control effects, coupling rates of major changes upstream to downstream, and creation of large scale terraces.

We began the mapping process by first executing careful observation and study of current aerial photography and soil profile maps. Landforms were located by identifying by such features as the sweeping curves of a channel cut bank or the tangled system of small channels branching away from a large channel, indicative of a splay deposit. The hypothesis we generated was then tested, to determine accuracy of assessed features such as splay deposits, channel fills, and point and longitudinal bars. The testing

we did included hand auguring boreholes with 10cm sampling intervals. We interpreted this data by examining logs of these holes looking for evidence to support the original hypothesis. A channel fill is expected as a sequence of heavy clay and silt coarsening downward into sand, as opposed to a point bar would be expected to be coarse sand within just a few meters of the surface. This data was recorded, accurately mapped through GPS, and geo-referenced on to the final map.