Combining a Flood Retention Structure with Ecosystem Enhancement on the Chehalis River Washington

The Chehalis River has flooded repeatedly, including one in 2007 that caused $1 B in damages and closed a major freeway (I-5). We are working with state, tribal and local agencies to evaluate the engineering, ecological, and economic effects of actions to control floods, protect I-5, and the potential effects of climate change on aquatic species and habitat enhancement for these species.  The project is unique in three ways:  1) the entire Chehalis River ecosystem is being evaluated; 2) maintaining habitat-forming processes in the river and fish passage was incorporated into dam alternatives; and 3) developing plans for enhancing the watershed for fish and amphibians is an integral component of the study. An analytical framework consisting of physical and biological models, both quantitative and qualitative, was assembled.  Physical changes are projected using a HEC-RAS hydrologic model coupled to a GIS elevation model to evaluate floodplain inundation, a water temperature model, and an evaluation of sediment movement and channel migration. These physical changes are being evaluated for impacts on coho and Chinook salmon and steelhead using the Ecosystem Diagnosis & Treatment model and Shiraz, a salmon life cycle model, and Habitat Suitability Indexes for other fish and non-fish species.