Elevations relative to the river surface were assessed using 2018 high-resolution LiDAR elevation data. A Relative Elevation Model (REM) was created by normalizing the elevation data set (LiDAR data) to approximate the trend of a water surface in the river using a cross section methodology (Figure 1). The resulting data display areas above the river grade as positive values and areas below the river grade as negative values as it extends laterally away from the river. 

To relate relative elevation to inundation risk, a review of stage discharge relationships for the three USGS Bighorn River gages was used to define a range of elevations relative to the LiDAR river elevations that may be 

at risk of inundation. Based on this assessment, elevations within two to three feet of the LiDAR river elevation are described as relatively high risk, three to six feet are considered moderate risk, and above 6 feet are considered low risk. Elevations at or below the LiDAR river elevation are assumed to be at very high risk of inundation. Areas greater than 10 feet above the LiDAR river elevation were determined to be above any likely inundation scenario and thus excluded from the maps. These risks are presented as a color gradient on the maps as shown in the map legend (Figure 2).

Figure 3 shows an area with a relatively narrow floodplain, but with some older channels with reactivation potential.