QUATERNARY GEOLOGIC MAPPING USING LIDAR DATA FOR A DYNAMIC SANDY RIVERSCAPE IN THE NORTHERN GREAT PLAINS: THE NORDEN AREA OF THE NIOBRARA NATIONAL SCENIC RIVER VALLEY CORRIDOR, NEBRASKA

As part of a USGS project on landscape response to climate change in the Greater Platte region, geologic mapping along the Niobrara National Scenic River in northern Nebraska is done in partnership with the National Park Service and the Nature Conservancy, as conservators of this area known for its unique ecological crossroads. Airborne Lidar data were acquired in November 2012 for an area of 68 square miles along the central part of the river valley corridor. The data acquisition was piggybacked on an adjoining, larger area of Lidar acquisition for NRCS. The new Lidar data enable more accurate and precise delineation of many landforms that record extensive postglacial geomorphic response to climate change in this dynamic and ecologically significant area of the Great Plains.

Mapping of several types of geomorphic features in this area is especially facilitated by analysis of Lidar data. Fluvial scarps that bound terraces are basic elements of fluvial landscapes and provide constraints on the history of erosion, incision and changing riparian environments during recent geologic and historic time. Scarps thus provide relatively sharp bounds to deposits of differing ages and potentially to polygonal map units, depending on how classifications are defined. Other geomorphic elements in which mapping is improved by Lidar data include valley/slope margins, distinction of alluvial types (tributary vs. riverine) and characterization of high terraces and eolian dune forms mantling uplands that adjoin the modern river valley.

Combined with analysis of historical airphotos, the above types of mapping were further classified relative to airphoto dates. Geologic mapping using Lidar typically involves toggling between various views and combinations of Lidar data within an ArcGIS platform: hillshades (with aspect bias), slope classification (for which aspect is NOT a factor); altitude by color spectra and contours; as well as other georeferenced data, including NAIP orthoimagery and field observations/ sampling sites, partly located with GPS. Our geologic mapping thus documents historic narrowing of the modern river system and riparian environments and tests for increased incision rates during historic time, among other aspects of environmental change.