Walla Walla means place of many waters. But at times in the past there was a bit too much water. Mill Creek which flows through Walla Walla has been heavily engineered mill-creek-and-walla-walla. In doing some research for work I came across some images from the State Archives that show the why of that engineering.
West Alder Street at S 2nd Street
The big flood took place in March 1931. The flood was a classic eastern Washington event: rain then snow then a very sudden warm up with a warm Chinook wind. Alder Street shown above is one and half blocks north of Mill Creek. Walla Walla is essentially built on the broad alluvial plan of Mill Creek. When the creek flooded in 1931 the flood waters were channeled down several of the main east-west streets of Walla Walla.
After the 1931 flood, a huge flood project was undertaken to reduce the flood hazard. This included a dam on Mill Creek, engineered diversions and construction of deep concrete channel for the creek through downtown including routing the creek through a tunnel. The work was completed by 1941 and subsequent floods have been contained.
View down West Alder Street
West Alder Street (I think)
South 7th Avenue and West Poplar