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Use of Floodscape Mapping to Assess Changes in Inundation Frequency of the Upper Mississippi River

As an alternative to probability-based maps, DeJager (and others) reconceptualized floodplain mapping using the notion of “floodscapes” (DeJager et al., 2015). A floodscape map is developed using daily stage data and high-resolution topography to create maps that depict the average number of days of inundation (per year or growing season). Floodscape maps reveal intricate and continuous patterns of high-frequency flows that are critically important to floodplain ecological functioning. DeJager found significant correlations between floodscape inundation and riparian vegetation patterns but single floodscape maps only provide snapshots in time. This current research develops and utilizes multiple floodscape maps to quantify changes in flooding over decadal time periods. Using a Mississippi River site near Keithsburg, IL, we developed floodscape maps for Blackhawk Island over three different 30-year time periods (1908-1937, 1939-1968, and 1986-2015). Historic floodplain topography was developed from 1938 1-ft contour-line (0.31m) topographic maps and modern digital terrain were developed from 2011 bare-earth LiDAR data. These maps quantify floodscape changes in the 73 years since the completion of the lock and dam system. When comparing 1938 to 2011, DEM differencing reveals a geomorphic pattern dominated by deposition and aggradation. Despite the increase in terrain elevation, this research finds an overall pattern of more frequent inundation (a more submerged floodscape) and demonstrates that increases in stage frequencies have overwhelmed the increases in floodplain elevation in the 73 years since the construction of the lock and dams. Floodscape-change analysis provides a useful spatial tool for visualizing and conceptualizing flood-inundation changes over time.