Bighorn River Channel Migration Zone Mapping
Jan 2021
study objectives
The objective of the Channel Migration Zone (CMZ) mapping is to use historic imagery to evaluate rates and patterns of channel movement on the Bighorn River. The ultimate product of the effort is a series of Channel Migration Zone maps that extend from Afterbay Dam to the Yellowstone River, accompanied by a report that describes methods and results. The project also has a public outreach component focusing on the agricultural, tribal, and recreational sectors along the river. The mapping can be used to assess patterns of bank erosion in areas of concern, locate old channel paths and restoration opportunities, and identify areas with spawning gravel recruitment, riparian expansion, and cost-effective land use management opportunities.
findings
Channel migration rates from the Afterbay Dam to the Yellowstone River confluence were measured using historic aerial photography from 1954 to 2017. Over 1,200 measurements of channel movement were made and attributed to reflect whether the erosion was into valley bottom alluvial materials or valley edge blufflines. The resulting vectors were then split at the 1979 bankline location to reflect pre- or post-dam movement.
Analysis of the results (Figure 2) shows a stark contrast between pre-and post-dam annual migration rates, with the pre-dam (1954 to 1979) rates being much higher than post-dam (1979 to 2017). This reflects the managed flow regime resulting from dam operations. Since the pre-dam rates do not reflect current river conditions, the Channel Migration Zone is based on the post-dam migration rates.
The resulting Channel Migration Zone mapping (Figure 3) shows a hierarchy of three key CMZ areas: 1) the most recent channel location (2017), 2) the Historic Migration Zone that is the cumulative footprint of all mapped channel locations, and 3) the Erosion Hazard Area that is a buffer placed on the most recent channel banklines that reflects the likelihood of 100 years of channel migration. LiDAR data will be used to identify areas prone to avulsion (excavation or reactivation of an old channel or swale) that extend beyond the footprint shown in Figure 3.
implications
The CMZ mapping should be primarily viewed as a tool to support informed management decisions throughout the Bighorn River corridor. Potential uses for the CMZ maps include:
Correlate channel migration patterns to aquatic resource health.
Identify specific problem areas where migration rates are notably high and/or infrastructure is threatened.
Strategically place new infrastructure to avoid costly maintenance or loss of investment.
Strategically place new infrastructure to minimize impacts on channel process and associated ecological function.
Improve stakeholder understanding of the risks and benefits of channel movement.
Facilitate productive discussion between regulatory, planning, and development interests active within the river corridor.
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