To assess historic river locations, migration, and conditions, banklines were digitized from the 1954/1956, 1979/1980, 1996,  2005, and 2017 imagery (Figure 1). Additionally, river channel centerlines, including secondary channels, were digitized from  the 1954/1956 and 2017 imagery High-resolution LiDAR elevation data was collected  in Fall 2018 by the NRCS for the Bighorn River  corridor from Yellowtail Dam to the confluence with  the Yellowstone River. These data were mosaicked  into a single Digital Elevation Model (DEM) and used  to create a Relative Elevation Model for the corridor  (Figure 2). The data is useful for assessing connectivity  of side channels, inundation potential, and avulsion risk  (Figure 3).  

Figure 3 displays a topographic cross section through  the Juniper side channel (see Figure 2 for cross section  location). The main river channel and the smaller  Juniper Channel, along with various perched historic  swales are clearly visible. These data are key for a variety  of tasks such as understanding channel connectivity  and potential for reactivation. 

The study area is divided into seven  defined reaches numbered upstream  to down. These reaches are used  to analyze changes in channel  morphology through time as defined  by the bankline and centerline  mapping. The following are some  preliminary findings of the analysis.  

1. Visual inspection of the imagery  indicates a clear loss of open bar area  and channel complexity through time.  

2. Bankline mapping shows a  consistent loss in both channel and  island area from the mid-1950s  to 2005, with a slight rebound in  2017. Figure 4 presents the bankline  mapping data for reach BH3 (Bighorn  FAS to Mallards FAS), showing the  loss of both channel and island area.  

3. Approximately 13.8 miles of  secondary and anabranching channel  length has been lost between the mid-  1950s and 2017 between the Afterbay  Dam and the Yellowstone River.