In 1931, a torrent of brown water roared down Mill Creek, with water levels rising at a rate of one foot per hour. The water escaped the confines of the riverbanks and rushed through the City of Walla Walla, destroying homes, roads and bridges. Born in the wake of this disastrous event, the Mill Creek Project now stands to protect the city from floods like this.
The Mill Creek Project is first and foremost a flood control project, constructed to protect the City of Walla Walla and adjacent lands from flooding. However, it is also a go-to place for recreation and leisure with their families.
Rooks Park, adjacent to the Mill Creek Diversion Dam, offers 18 acres of project lands for families to play or picnic. Then there is Bennington Lake, a magnet for fishermen and boaters during the warmer months. For hikers and horseback riders, there are more than 20 miles of recreation trails throughout the Mill Creek Project lands. Wildlife also finds a protected haven, as the habitat at the project has been developed by the U.S. Army Corps of Engineers, the State of Washington and local volunteers.
A history of flooding
Before the Mill Creek Project was conceived, flood risk in the region was managed locally using intermittent concrete retaining walls bordering both sides of the channel, mostly within the Walla Walla city limits.
With its headwaters in the heavy snows of the Blue Mountains, Mill Creek is subject to wide fluctuations in flow, which regularly lead to flooding.
On March 31, 1931, an estimated 6.65 inches of intense rainfall created what was termed “the worst flood in Walla Walla’s history.” The floodwaters resulted in extensive damages throughout the city.
This flood, and concerns over the reoccurring nature of Mill Creek’s floods, prompted Virgil B. Bennington, then President of the Walla Walla Chamber of Commerce, to lobby the U.S. Congress for flood protection. The project was authorized by the Flood Control Act of 1938.
The Mill Creek Flood Control Project consists of the Diversion Dam, intake canal, Bennington Lake, Mill Creek Storage Dam, Russell Creek canal, Mill Creek return canal, and the Mill Creek Channel. Bennington Lake was originally known as Mill Creek Reservoir or Mill Creek Lake. It wasn’t until 1992 that the lake was renamed to honor the late Virgil B. Bennington.
Construction of the dam and its associated works was completed in 1942.
When flows through Mill Creek rise too high, the Diversion Dam diverts the excess, via the intake canal, into Bennington Lake, which is impounded by Mill Creek Storage Dam. Mill Creek Storage Dam is an earth-fill structure with a heavy gravel face. It is 800 feet wide at the base, 125 feet high and 3,200 feet long at the crest. After a high-water event, water in Bennington Lake is released via the Russell Creek canal and the Mill Creek return canal. Water that is not diverted into the lake travels down the Mill Creek Channel, through the middle of the city.
The paving of the Mill Creek Channel through the City of Walla Walla was completed in 1948.
A new district
During the completion of the Mill Creek Project, wheels were turning within the U.S. Army Corps of Engineers that would change the history of the project forever.
At the time of its construction, the land surrounding the Mill Creek Project was under the jurisdiction of the USACE Portland District. In the 1930s, the boundary of the Portland District extended east across the majority of Idaho, encompassing small portions of Nevada, Utah, and Wyoming as well.
However, the region was quickly developing. In 1945, the River and Harbor Act authorized five dams, which would later become McNary Lock and Dam and the four Lower Snake River Dams. The very next year, the River and Harbor Act authorized a flood control project in Boise, which would become Lucky Peak Dam and Lake.
All these projects, plus the future work that was envisioned, prompted USACE to consider establishing a new district. A survey was conducted in 1947 of several towns, including Pendleton, Tri-Cities, Spokane, Boise and Walla Walla, for the best location of a possible district office. Ultimately, the City of Walla Walla was chosen, and the Walla Walla District was established on Nov. 1, 1948.
The location of the freshly minted Mill Creek Project does not seem to have played a role in the selection of Walla Walla. However, the project was indeed passed to the new district, along with two levee systems that were currently under construction: one in Milton-Freewater, Oregon, and one in Heise-Roberts, Idaho.
Under the Walla Walla District, the Mill Creek Project became more than a flood control project. In 1965, recreation was added to the project purposes through the Federal Water Project Recreation Act. While this addition has shaped how the project operates, reducing the threat to lives and property through flood control remains its highest priority.
The 1996 flood
In early February of 1996, temperatures within the snow-covered Mill Creek Watershed rose from minus 25 F to 60 F and over 1 inch of rain fell. The frozen ground, melting snow, and rainfall culminated in a flood that put the Mill Creek Flood Control Project to the test.
Walla Walla District project operators began diverting water into Bennington Lake when river flows reached 1,400 cubic feet per second(cfs). Flows through town increased incrementally from 3,500 cfs to approximately 3,800 cfs, surpassing the designed capacity of the channel, as storage capacity in the lake decreased.
Minor flooding occurred within Walla Walla, but major damages were avoided. Bennington Lake was filled to 97% of capacity.
The 2020 flood
24 years later, almost exactly to the day, Mill Creek experienced another historic flood.
During the week of Feb. 6, 2020, a developing "atmospheric river" began pushing large amounts of moisture into the Pacific Northwest. Over 8 inches of rain fell in 60 hours. The resulting flood set a new peak flow record at the Kooskooskie gage (11 miles upstream of the project) of 7,050 cfs. Bennington Lake was filled to 80% and was in within 11 hours of reaching maximum capacity.
Across the Walla Walla Valley and surrounding areas, the historic flows resulted in a multitude of damages. However, thanks to the Mill Creek Project, the City of Walla Walla was not flooded.
Preparing for the future
The 1931, 1996 and 2020 floods have become historic markers, however, there have been other flood events in between, of various sizes. Between 1878 and 1931, Mill Creek flooded the City of Walla Walla 15 times. Walla Walla means the “place of many waters” and Mill Creek helps the region live up to that name on a regular basis.
One of the most recent events was on June 13, 2022, when heavy rainfall led Mill Creek project operators to divert water into Bennington Lake to minimize flows through downtown Walla Walla. Diversions stopped at 5:45 a.m. on June 14 and Bennington was only filled to 10 feet above recreation levels.
The Mill Creek Project has been the crux of flood protection for the City of Walla Walla for 80 years. Recognizing its importance, the Walla Walla District, in partnership with Walla Walla County, began a General Investigation study on the Mill Creek Flood Control Project in 2018.
The final report for the study was approved by Northwestern Division on Sept. 2, 2021 and included a recommended plan for improvements to the Mill Creek Flood Control Project.
The recommended plan, now known as the Mill Creek Section 205 Project, includes the following actions:
- Increase levee heights by up to 2.2 ft in low assurance areas along Mill Creek, to increases the system’s capacity.
- Repair and reinforce the concrete channel through downtown Walla Walla to improve the reliability of the flood control project.
- Increase the floodwater diversion trigger from 1,400 cfs to 1,700 cfs to save space in Bennington Lake for larger or subsequent flood events.
On Nov. 21, 2022, the Walla Walla District and the Mill Creek Flood Control Zone District signed a Project Partnership Agreement for the Mill Creek Section 205 Project.
The agreement defines the shared responsibilities and cost-sharing commitments of the Walla Walla County and the U.S. Army Corps of Engineers as they jointly design and construct improvements to the Mill Creek Flood Risk Management System over the next few years.
Design for the project is expected to begin in 2023, with most construction taking place in 2024 and 2025. The project is expected to be completed by early 2026.
For years to come
The Mill Creek Project is over 80 years old, older than the Walla Walla District, which will be celebrating its 75th Anniversary on Nov. 1, 2023. Since construction was completed in 1942, at least $300 million in potential flood damages have been prevented by the Mill Creek Project’s combined storage and channel operations.
Since its construction, the project has grown to be more than a flood control system. Now it is a place where, every year, approximately 100,000 visitors picnic, hike, boat, fish and play. It has become a focal point of the community it protects, and it will continue to serve for years to come.