Spatial Imagery Consolidation and Channel Feature Delineation
This study consisted of three primary tasks, with one additional task (#4) added during the study:
Identify and compile available data and aerial photography for use in GIS.
Map banklines and physical features.
Analyze data and summarize results.
Create a Bighorn River Atlas.
The resulting data and maps provide the spatial context for assessing historic and current conditions throughout the river corridor.
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.
Visual inspection of the imagery indicates a clear loss of open bar area and channel complexity through time.
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.
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.
The work completed under this part of the Research Initiative provides a foundation for future research efforts related to aquatic and floodplain habitats, channel stability, bank erosion, armoring effects, side channel evolution, and agricultural land loss. The data provide some historic context regarding longer-term system evolution as well as a modern snapshot of river conditions. The results are intended to help guide the development of effective management/restoration strategies as well as identify optimal locations for specific project implementation.