Background – Mill Creek is a tributary of the Walla Walla River, draining a 96-square-mile watershed. It drops about 430 feet in elevation from its headwaters in the Blue Mountains, through 7 miles of the existing flood-risk-management system, on its way to the river. Mill Creek flooded the City of Walla Walla, Washington, 15 times between 1878 and 1931, including the disastrous flood of 1931. The constructed channel reaches are part of the Mill Creek Flood Control Project, which was authorized in 1938. The city’s economic security and the safety of its citizens depend on the system’s four main components: (1) Storage dam, which impounds Bennington Lake, an off-channel flood storage reservoir. (2) Mill Creek diversion dam, which provides the head required to divert water through a channel into Bennington Lake. (3) Leveed channel (two sections), upstream and down-stream of the concrete-channel section of the system. (4) The concrete channel, which runs through and under the City of Walla Walla, beneath businesses, city streets and state roadways, parking lots and historic buildings. It was constructed by the Works Progress Administration in the 1930s using unknown materials and methods, and was later incorporated into the Corps project upon completion in 1948. Vulnerabilities – The age and deterioration of the levees, concrete channel and infrastructure located above or alongside the channel are of great concern to the community of Walla Walla. The channel was designed in the 1930s to handle flows of up to 5,400 cubic feet per second (cfs). However, a 1996 flood of 4,100 cfs (75 percent of the design flow) damaged the leveed portions of the system, flooded upstream and downstream of the levees and damaged parts of the concrete channel, sometimes nearly overtopping the channel.

Mill Creek Flood Risk Management General Investigation Study

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 Update October 2021

The final Mill Creek General Investigation Flood Risk Management Feasibility Study/Environmental Assessment Report was approved on September 2, 2021.  The final FS/EA Report links can be found on the right side of this screen.  Because the estimated cost of the Recommended Plan did not meet the General Investigation Program cost parameters, the action was transferred from the General Investigation Program to the Continuing Authorities Program (CAP).  The Corps Walla Walla District will now seek funding through the CAP to complete the Planning, Engineering, and Design Phase of the Recommended Plan.  This effort, as well as the subsequent Construction Phase, will be cost-shared 50/50 with the sponsor (Walla Walla County). 

 Update January 2020

*** Public Comment Period extended to Feb. 28 ***

WALLA WALLA, Wash. – The U.S. Army Corps of Engineers, Walla Walla District, invites public comments on the draft Finding of No Significant Impact (FONSI) and Integrated Feasibility Study and Environmental Assessment Report (FR/EA) for the Mill Creek Flood Risk Management General Investigation Study, Walla Walla, Washington. 

The Corps and the non-Federal Sponsor, Mill Creek Flood Control Zone District (Walla Walla County), are conducting the study to address the flood risks Mill Creek presents to the community of Walla Walla, Washington, and other communities within the Mill Creek watershed (adjacent lands).  This draft FR/EA documents the results of the feasibility study and environmental compliance processes.  It presents the process, analysis, results, and preliminary conclusions of the investigations and analysis performed to evaluate potential improvements to the existing Mill Creek Flood Control Project (MCFCP or Project).

The public comment period will begin on January 15, 2020 and conclude thirty days later on February 14, 2020.  Public comments may be submitted electronically to the Corps using the online comment form on the Environmental Compliance webpage, or via email -- type “Mill Creek GI” in the subject line. Comments must be postmarked or e-mailed no later than February 14, 2020 to be considered as part of the public record.  Comments should be mailed to:

U.S. Army Corps of Engineers, Walla Walla District
ATTN: CENWW_PPL-C, Mill Creek Study 
201 North 3rd Ave.
Walla Walla, WA 99362-1876


A public meeting is planned for Wednesday, February 12, 2020 at 5:30 p.m. at the Courtyard by Marriott, 550 West Rose Street in Walla Walla, Washington.  At this meeting, the Corps will give a presentation, conduct a question and answer session, and accept written comments from interested parties.

The draft FONSI and FR/EA are available in electronic form through the Corps’ website at

If you have questions, please write or e-mail the Corps at the addresses above, or call the project Environmental Compliance Specialist at 509-527-7256.


The U.S. Army Corps of Engineers, Walla Walla District (Corps), in partnership with its non-federal sponsor, Walla Walla County, proposes to conduct a general investigation study on the Mill Creek Flood Control Project (Project) in Walla Walla, Washington.  The study includes a modernized reassessment of the overall flood risks Mill Creek presents to the Walla Walla community.  The study will consider Project capacity, performance, and reliability of several components through a range of alternatives that could be implemented to reduce risks to the community looking forward. 

The Project was authorized in 1938 with the single purpose of reducing flood risk to the City of Walla Walla and surrounding communities.  

The existing Project consists of four main components:  

1) Bennington Dam, which impounds Bennington Lake, an off-channel reservoir 
2) Diversion Dam, which diverts flood water through a channel into Bennington Lake
3) Leveed Channel (upstream and downstream of Walla Walla), 
4) The concrete channel through downtown Walla Walla.  Since the disastrous 1931 flood, life safety risks have increased.  

The Project used to be able to divert floodwaters to Yellowhawk and Garrison Greeks. 

The flood risk reduction provided by the Project may be insufficient to address current risks, and the number of buildings in the flood zone has tripled and now includes five elementary schools, two middle schools, two high schools, two colleges, six nursing and assisted-living homes, and three hospitals.

A public scoping period was conducted Oct. 15 through Nov. 16, 2018, including a public meeting held Nov. 1, 2018, in Walla Walla, Washington. Information and comments submitted during the scoping period will be included in the permanent public record, and used to identify issues to address in the study, exploring alternatives and identifying potential environmental effects of possible changes to the system. The study process will include additional outreach activities with citizens and stakeholders across the community to gather their thoughts and ideas related to the way we could manage flood risks and other important water resources considerations.

The Mill Creek GI study will also include coordination and consultation with other federal and state agencies to find ways to minimize potential effects to valuable environmental resources, such as ESA-listed fish, water quality, and cultural and historic resources.

The Corps is committed to working with our non-federal sponsor, Walla Walla County, and the larger community to provide safe and sustainable solutions to reduce flood risk in the City of Walla Walla and surrounding communities.

We will keep the public informed by updating this webpage and distributing information to area news media as the study progresses.  

 What is a General Investigation/ Feasibility Study

Contact Us

Environmental Compliance
PPL-C, Mill Creek GI
201 N. 3rd Ave.
Walla Walla, WA 99362