DESCRIPTION
Mill Creek is a Mid Columbia priority for Steelhead Recovery in the Walla Walla basin. The SRSRB, CTUIR and WDFW have been supporting the Tri-State Steelheaders in the improvement of fish passage through the City of Walla Walla upstream through the Mill Creek flood control structure built and regulated by the USAOE. The reach 4 section of the flood way consists of a concrete floor and a center dividing wall creating two nearly equal flow paths. Fish passage is limited through this reach both by high velocity during high flows and inadequate water depth during low flows. The reach has not had a design up to this pint and will require one going forward prior to construction. The reach is ~ 500 ft in length and is located between Paluse St and Park St in downtown Walla Walla. This reach type exists in two other location: one is located under the 5th Street bridge and the other under the 6th Street Bridge in the downstream end of the flood way.
A flood control project on Mill Creek at Walla Walla includes a diversion dam, storage reservoir, and a 6.7 mile long flood control channel, which includes 263 channel spanning weirs (sills) and and a concrete flume.
The flood control channel creates water quality problems and presents a series of complex fish passage barriers. ESA listed steelhead, and bull trout, and reintroduced spring Chinook utilize the flood control channel during migrations. Returning adults encounter flow-dependent passage problems associated with channel spanning weirs, light attraction barriers (in the underground sections), and a lack of resting water in the concrete channel section, among other problems. Juvenile fish encounter low spring flows, and high water temperatures in late spring. Often by mid-May adults and juveniles become trapped in the flood control channel where they experience lethal temperatures. Many of these passage issues are considered as imminent threats in the Snake River Salmon Recovery Plan. Mill Creek, upstream of the flood control project, is a critical and under-utilized area for spawning and rearing of ESA listed species, and provides for an important recovery opportunity for those listed fish, as well as good habitat for other native fish and reintroduction efforts for spring Chinook.
This project will build on the design and flow model tests produced in the Mill Creek Barrier Assessment completed in 2009 and scale model tests to provide fish passage through the concrete flume section of the flood channel. Fish passage alternative have been identified in the assessment and include the development of resting pools and increased roughness through much of the existing channel. Reach Type 4 is short in length totaling 180 feet and is defined by trapezoidal flume with split baffels. The conceptual restoration action would likely be similar to other aproaches by increasing over bank roughness an increasing pool frequency.
Walla Walla River Population
