Fish Passage Assessment and Design in Mill Creek

Mill Creek is a tributary to the Walla Walla River, and flows through the city of Walla Walla, Washington.  In the 1930s, after enduring several large floods, the people of Walla Walla, led by Virgil B. Bennington, started a petition for federal funding to build flood control structures to protect the City.  Following approval by Congress, President Roosevelt signed the Flood Control Act of 1938 in June of that year.  The Act called for two flood control projects to be built by the Corps, the Mill Creek Project and the Mill Creek Channel.  The Mill Creek Project includes two dams, a mile of Mill Creek between the dams and a storage reservoir, and surrounding lands. Bennington Dam (or Diversion Dam) at river mile (RM) 11.5 is the uppermost of the two dams. Its purpose is to divert flood flows up to 5000 cfs into the reservoir where the water is stored until it can be safely discharged.

The Mill Creek Channel continues downstream from the Division Dam Head Works at RM 10.6, to its end at RM 4.8.  The Channel consists of concrete channel-spanning stabilizers and a concrete flume.  This concrete flume runs through the City of Walla Walla where it then transitions back to the channel spanning sills.  The Mill Creek Channel is owned by the Mill Creek Flood Control Zone District which is directed by the County Commissioners and is responsible for the normal operations and maintenance of the Channel.

In 2008 the Tri State Steelheaders Fisheries Enhancement Group, the Department of Fish and Wildlife and other local stakeholders were awarded a Salmon Recovery Funding Board (SRFB) Grant to complete a detailed fish passage assessment report.  The study identified the complexity of hydraulics and fish passage issues and calculated a percent passage for 12 different reaches.  Key to the analysis was the development of a fish passage energetics model which calculates the energy expended by fish attempting to pass through the flood control channel.  A preferred design was selected from an alternatives analysis.  A 1:8 physical model was then tested in Seattle by Northwest Hydraulics.  Features of the model included a roughened channel, plunge pools, resting pools and baffles.