The Mill Creek Flood Control Project is infrastructure built by the Corps of Engineers in the 1940s. It has saved Walla Walla many times from flooding, most recently in 2020. In this case, the stream literally is infrastructure that is essential to the safety of the community.
However, the flood control channel is a barrier to over 50 miles of headwater habitat for salmon, steelhead, and bull trout. Beginning in 2011, five construction projects have improved passage in a two-mile long concrete section of the channel.
In September 2021, passage improvements were applied to a 2,200 foot long section of the channel. This involved concrete cutting and demolition, rebar tying, and forming and pouring of new concrete. The completed work changes the depth and velocity of water in the channel so that it is passable for adult and juvenile salmonids.
To date, nearly half of the concrete channel has been improved for passage.