The project was authorized by the Flood Control Act of 1938. Recreation was added to the project purposes through the Federal Water Project Recreation Act of 1965.
Construction of the dam and its associated works was completed in 1942. An auxiliary outlet channel from the dam to Russell Creek and the construction of additional drainage facilities at the toe of the dam were completed in 1944. The sealing of the lake bottom, additional work on the drainage system in the foundation and the installation of an upstream outlet gate were completed in 1950. The paving of Mill Creek Channel through the city of Walla Walla was completed in 1948.
The project consists of the Mill Creek Channel, Bennington Lake and associated federal lands. The dam and reservoir portions of the channel and lands are operated and maintained by the U.S. Army Corps of Engineers (USACE). The project provides for flood risk management, recreation, wildlife habitat and irrigation. Since 1942, almost $75 million in potential flood damages have been prevented by Mill Creek project’s combined storage and channel operations.
Mill Creek Dam
The dam is an earthfill structure with a heavy gravel face located in a drainage off Mill Creek. The dam is 800 feet wide at the base, 125 feet high, 20 feet wide at the top and 3,200 feet long at the crest. A 2260-foot-long concrete cutoff wall extends 2 feet into bedrock.
This off-stream reservoir has a maximum storage capacity of 8,300 acre-feet at elevation 1265, with a 5-foot freeboard. The reservoir is the only public lake within 45 miles of the city of Walla Walla.
Mill Creek Channel
About 5,000 feet of the Mill Creek channel is federally operated and maintained. The locally operated portion of the channel begins at the Mill Creek-Bennington Lake unit and extends six miles through the city of Walla Walla.
The diversion facilities consist of a diversion levee, diversion dam, debris facilities and intake canal facilities. The levee is a rolled earthfill dam, 1,700 feet long and 20 feet high. The diversion dam contains an Ambursen Ogee-Crest type spillway and outlet. It is 250 feet long and 14 feet high.
Division Works & Return Facilities
The division works allow water to be divided between Mill Creek, Yellowhawk and Garrison Creeks. The return facilities consist of the outlet works and two outlet canals.
There are two fish ladders that provide fish passage in the Mill Creek Channel. Large boulders were placed in the channel to provide resting places for migratory fish. In 2001, fish screens were installed at the intake on the diversion structure to prevent trapping fish in Bennington Lake. In 2008, fish screens were installed at the mouth of Garrison Creek to enhance fish migration in Yellowhawk Creek.
There are about 611.5 acres of federally owned lands and 87.3 acres of easement lands. This is the largest public open space in the Walla Walla Valley. These lands provide flood risk management, project operation, and recreation and wildlife benefits. There are recreational facilities at Rooks Park, Mill Creek Trail and Bennington Lake Recreation Area. There are more than 20 miles of recreation trails throughout Mill Creek project lands. Wildlife habitat has been developed by USACE, the State of Washington and local volunteers. Mill Creek hosted 95,803 visitors during 2020.
Fewer than 10 full-time Walla Walla District employees work at the Mill Creek project. They serve as park rangers, natural resource specialists, administrative staff and maintenance workers. Temporary employees augment the staff during high-visitation months. Together, they provide the safe and continuous operation of the project. The staff also manages a robust volunteer program to help maintain and make improvements to Mill Creek project’s numerous public recreation facilities.