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.

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