1940s Mill Creek Project helped divert water from downtown Walla Walla during historic 2020 flooding

WALLA WALLA, Wash. — When historic flooding hit the Walla Walla Valley last week, it was a decades-old Army Corps of Engineers project that helped prevent an even greater disaster.

“Everything on project worked as designed when we needed it to,” said Justin Stegall, Mill Creek Project Manager. “The integrity of everything here is very solid.”

Major flood events occured in Walla Walla in 1931, 1964 and 1996. The Mill Creek Project was authorized by Congress in 1938, requested by the Army Corps of Engineers Walla Walla District after the floods of 1931 severely damaged the downtown area. Various parts of the project were constructed through the forties and completed by 1948.

The Mill Creek Project, which currently has a full-time staff of just ten people, involves a reservoir, diversion dam and series of canals that can manipulate floodwaters, moving them away from the city center.

There are many smaller details of each of the pieces of infrastructure that allow it all to work smoothly. For example, debris barriers sit at the top of the diversion dam to keep tree trunks and other items from flowing down into Mill Creek, and ultimately, downtown. After the flooding, crews took away 900 truck loads of debris that had been caught by those barriers.

Many who had lived in Walla Walla since the 90s compared this year’s flooding to the floods in 1996. Corps officials analyzed and compared the two events.

According to Corps data, 1996 and 2020 both had two peaks, and they also occurred around the same time of year. However, the 2020 floods brought more intensity in a shorter period of time, while the 1996 floods were prolonged over about 80 hours.

Last week’s floods were a 48-hour event, lasting from Feb. 6 to Feb. 7. During that time, Corps officials continuously monitored water flows up and down the Mill Creek Channel and throughout the Walla walla Basin.

Thursday night they increased diversions from Mill Creek into Bennington Lake, and at one point, close to 3,000 cfs (two feet per hour) were being sent into the lake.

The lake reached 80 percent capacity by Friday afternoon. At that time, Corps officials said if the rain had continued at its current intensity into late Saturday they would have run out of capacity at Bennington Lake in about 12 hours.

To the relief of the community and those monitoring the floods, the rain subsided by Saturday. By the afternoon, Rooks Park – which sits along the Mill Creek Channel – was reopened to the public.

“It was really neat to have planned and talked about these things and then see everything happen the way we talked about it and the way we planned for it,” said Stegall. “ It was just amazing.”

Wednesday morning, crews started releasing water out of Bennington Lake through the return canal and Russell Creek. As of Wednesday, parking signs and part of the parking lot at Bennington Lake were still still under water, along with the intake tower.

In the days ahead, Corps officials said they will work on draining the lake and restoring parking lots and walking paths to the community.