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Mill Creek Dam and Bennington Lake

Mill Creek Dam
The dam is an earthfill structure with a heavy gravel face located in a drainage off of 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.

Bennington Lake
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.

Project Information

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Authorization
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.

Progress

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.

Project
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 Corps. 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 of 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 260 foot long concrete cutoff wall extends 2 feet into bedrock. 

Bennington Lake
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.

Diversion Facilities
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.

Fish Passage
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.

Lands
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 the Corps, the State of Washington and local volunteers. Visitation during fiscal year 2015 was more than 360,000.

People
Less 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.

Budget
Fiscal year 2015 total expenditures were about $2.7 million for the Mill Creek Project.



 

General
Official Name: Mill Creek, Washington*
U.S. Army Corps of Engineers Reference: Mill Creek Project

Location:
State - Washington
County - Walla Walla
Stream - Mill Creek

Construction Completion Dates:
Dam and appurtenant works - 1942
Mill Creek Channel - 1949

Owner: U.S. Government
Managers: U.S. Army Corps of Engineers and Mill Creek Flood Control Zone District
Authorized purposes: Flood control and recreation
Type of Project: Channelization and off-stream storage
**Real Estate: 611.46 acres of fee lands and 87.27 acres of easement lands

Federally-Owned Units
Diversion Works
Diversion Dam:

Spillway
Type - Ambursen, ogee crest
Length at crest - 250 ft
Crest elevation - 1261
Height - 14 ft
Design discharge, cs (with water surface elevation 1268) - 17,000
Concrete structure top elevation - 1270
Stilling basin length - 24 ft
Stilling basin invert elevation - 1245
Type - Radial sluice gate
Size - 6x8 ft
Number - 1
Sill elevation - 1247
Control - Manual, with portable engine drive
Maximum allowable discharge - 400 cfs

Fish Ladder:
Width - 6.5 ft
Capacity - 42 cfs
Operating range elevation - 1253 to 1256
Intake invert elevation - 1250.25
Exit invert elevation - 1245
Stilling Basin:
Length - 4 ft
Width - 19.5 ft
Floor elevation - 1242
End sill elevation - 1244

Diversion Levee:
Type - Earthfill with heavy gravel face
Crest elevation - 1270 to 1280 ft
Length at crest - 2,200 ft
Top width - 12 ft
Maximum height - 23 ft
Design freeboard (standard project flood) - 5 ft

Debris Facilities
Debris Barriers:
Location - Diversion Dam forebay
Length - 550 ft
Type - Steel crib and cable

Shear Wall:
Location - Headworks Intake Canal
Length - 90 ft
Type - Panel

Intake Canal Facilities
Headworks:
Type - Concrete non-overflow with radial gates
Gate size – 8x18 ft
Number - 4
Sill elevation – 12,525
Control - Manual (optional use of portable electric operator)

Canal:
Intake canal end, elevation - 1,250
Invert elevation - 1,252
Capacity - 7,000 cfs
Intake canal base width - 80 ft
Intake canal length - 1,800 ft

Off-Stream Storage Reservoir (Virgil B. Bennington Lake)
Name: Virgil B. Bennington Lake***
Maximum pool elevation for flood control - 1,265
Capacity at elevation 1265 – 8,300 acre-feet
Maximum allowable time for storage above elevation 1235 (due to stoppage) - 15 days
Capacity at elevation 1235 – 3,300 acre-ft


Storage Dam (Mill Creek Dam)
Type: Earthfill with heavy gravel face
Crest elevation - 1,270
Length at crest - 3,200 ft
Top width - 20 ft
Height above valley floor - 1,150 ft
Toe of embankment, elevation - 1,215
Maximum width at base - 800

Embankment Toe drains:
Date nine wells rehabilitated, year - 1,979
Drainage discharge header, elevation - 1,135
CP manhole diameter - 48 in

Outlet Works
Intake Tower:
Slide gate, centerline elevation - 1179
Intake tower, weir overflow elevation - 1212
Lower sluice gate, centerline elevation - 1189

Beneath Dam:
Type - Steel pipe
Diameter - 42 in
Length - 900 ft
Discharge pipe, elevation (varies) - 1147.5 to 1181

To Mill Creek Return Canal:
Valve type - butterfly valve
Diameter - 42 in
Length - 460 ft
Invert elevation at discharge end - 1210

To Russell Creek Canal:
Pipe Diameter - 36 in
Length - 125 ft
Howell-Bunger valve, elevation - 1147.5

Outlet Canals
Mill Creek Return Canal:
Type - Trapezoidal
Slope - .0008
Lining - Shotcrete
Hydraulic capacity - 190 cfs
Invert elevation at discharge end - 1210 ft

Russell Creek Canal:
Type - Trapezoidal
Slope - 0.01
Lining - Concrete
Hydraulic capacity - 250 cfs
Howell-Bunger valve elevation - 1147.5

Division Works
First Division Works
Mill Creek:
Gate type - Vertical lift gate
Size of opening:
Total width of openings - 97 ft
Height - 6 ft
Channel capacity - 3,500 cfs
Barrier height - 2 ft

To Yellowhawk-Garrison Canal:
Gate type - Radial lift gate
Total width of openings - 14 ft
Height - 6 ft

Fish Ladder
Operating Elevations:
Width - 8
Ladder design capacity - 15 cfs
Slope - 0
Entrance invert elevation - 1170
Exit invert elevation - 1170

Second Division Works
Yellowhawk Creek:
Ungated - 60
Channel capacity - 60 cfs

Garrison Creek:
Gate type - Slide gate
Channel capacity - 10 cfs

Mill Creek Flood Control Zone District Units
Gose Street to Mullan Avenue:
Type - Riprapped levee
Length - 1.9 miles
Capacity - 3,500 cfs

Mullan Avenue to Roosevelt Street:
Type - Concrete-lined
Length - 2.2 miles
Capacity - 5,400 cfs

Roosevelt Street to Diversion Dam:
Type - Riprapped levee
Length - 2.8 miles
Capacity - 3,500 cfs

Hydrologic Data
5-year flood event, natural - 2,000 cfs
5-year flood event, regulated - 1,470 cfs****
100-year flood event, natural - 7,050 cfs
100-year flood event, regulated - 3,500 cfs
Standard project flood - 11,300 cfs
Largest flood, 1931 - 6,000 cfs
Mill Creek drainage basin above Mill Creek at Walla Walla stream gage - 96 sq miles


References
*Authorizing legislation is Public Law 75-761, as amended by Public Law 77-228.
**These figures represent data from the Walla Walla District's Geographic Information System. Legal real estate documents may vary slightly.
***For the purpose of consistency with existing Mill Creek Project documents, the use of the terms "pool," "reservoir," or &qout;lake" is interchangeable.

The body of water currently known as Virgil B. Bennington Lake has, in the past, also been referred to as "pool," "Mill Creek Reservoir," and "Mill Creek Lake."

****1,400 cfs in Mill Creek and 70 cfs in Yellowhawk and Garrison Creeks.

Contact Us

Mill Creek and Bennington Lake
3211 Reservoir Road
Walla Walla, WA 99362-8400
Phone: 509-527-7160

Mill Creek Recreation

mill creek