Dams hold back sediment and can therefore impact downstream fish habitats. Our study on the Sultan River found that hydrology, and not sediment supply, is responsible for changes in the river channel and fish habitat downstream of Culmback Dam. We came to this conclusion by comparing contemporary river conditions in the Sultan River below Culmback Dam with historical conditions since 1964 using three methods:
1. Comparison of aerial photographs taken at three different time periods to understand changes in river shape and function over time
2. Comparison of sediment inputs downstream of Culmback Dam under pre- and post-dam scenarios, using digital landslide mapping from the State of Washington’s Landslide Hazard Program enhanced with field studies, a literature review, and our own GIS analysis
3. Sediment transport modeling comparing historical and current rates
We concluded that current sediment supply and transport rates are similar to what they were in the past, and that the channel form and shape have adjusted and therefore measures to correct sediment routing through the system would have little overall benefit for salmon. Our client was able to use this information to modify flows and develop mitigation measures for aquatic habitat to have maximum environmental benefit while maintaining necessary energy production.
These studies were the basis of several PM&E measures developed for the new license, expected in 2010. Stillwater will lead the design and monitoring of large wood placements and side channel enhancements to satisfy license stipulations.