A flood control project completed in the 1940s on Mill Creek resulted in over seven miles of modified channel, including a two-mile section of concrete-lined channel running through downtown Walla Walla.
Upper Mill Creek has over 40 miles of good to high quality spawning and rearing habitat. Fish managers believe that Mill Creek should produce higher numbers of fish, and passage in the flood control channel was thought to be contributing to low production.
An assessment in 2009 found the concrete channel poses complex, flow-dependent passage problems. At high flow, velocities are too high, and there are no resting areas. At low flow, the water is too shallow. This project removed existing concrete and replaced it with “roughness panels” and resting pools. Roughness provides a zone of swimmable velocities for adult summer steelhead, bull trout, and spring chinook. Resting pools provide low-velocity water where fish can recover energy.
The layout of channel baffles was also changed, providing backwatering that eliminates the depth problem. While these passage corrections will improve the number of spawning adults reaching upper Mill Creek, the project also addresses the spatial distribution component of recovery goals.