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Mann Creek Project
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Mann Creek Project History (36 KB) (pdf)
General Description| Plan| Development| Benefits

General Description
The Mann Creek Project in west-central Idaho consists of approximately 5,100 irrigable acres utilizing an existing distribution system in the narrow valleys of Mann and Monroe Creeks, both tributaries of the Weiser River. The natural flow of Mann Creek historically has been near its lowest point during the growing season when the demand for irrigation water is at its highest. Project development provides for storage of winter and spring flows of Mann Creek for use later in the irrigation season.

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Development of the Mann Creek Project consists of the Mann Creek Dam and Reservoir, control structures on Joslyn Ditch and Mann Creek Ditch, and rehabilitation of 4.1 miles of Lolley Ditch. Basically, the existing diversion, distribution, and drainage systems within the project areas are used. Mann Creek Reservoir furnishes a supplemental irrigation water supply for the irrigable lands within the Mann Creek and Monroe Creek areas of the project. At the request of the water users, the name of Mann Creek Dam and Reservoir was approved August 26, 1967. Prior to that, it was called Spangler Dam and Reservoir. In addition to irrigation, recreation, and fish and wildlife benefits, flood control benefits are being realized through the operation of Mann Creek Reservoir.

Facility Descriptions

Mann Creek Dam and Reservoir

Mann Creek Dam, on Mann Creek about 13 miles northeast of Weiser, Idaho, is a zoned earth and rockfill structure with a crest length of 1,176 feet and height of 148 feet above bedrock. The crest width of 30 feet with guardrails along each shoulder provides a crossing for the relocated Monroe Creek road. A reinforced concrete morning-glory spillway inlet that connects to a reinforced concrete conduit, 11 feet in diameter, terminates in a concrete stilling basin located along the right abutment. The outlet works, located along the left abutment, is also of reinforced concrete. The flow of water is controlled by two 2.25-foot-square high-pressure gates located in the control house and one 2.75-foot-square high-pressure gate located in the gate chamber for emergencies. Mann Creek Reservoir, at the normal water surface elevation of 2,889.0 feet, is 1.8 miles long, has a surface area of 283 acres, and at closure had a total capacity of 12,536 acre-feet (active 11,141 acre-feet).

A 1992 sedimentation survey estimated that Mann Creek Reservoir now has a total capacity of 12,536 acre-feet (active 10,917 acre-feet).

Distribution System

Both the Mann Creek and Monroe Creek areas have operating distribution systems which are being used in conjunction with Mann Creek Reservoir. These systems consist of several diversion structures across the two creeks, and ditches to serve the farmlands. The only work performed on these facilities was construction of headwork control structures on Joslyn Ditch and Mann Creek Ditch, and rehabilitation of 4.1 miles of Lolley Ditch. Existing Barton Reservoir, with a storage right of 2,000 acre-feet, is filled from Monroe Creek diversions through the Monroe Creek feeder canal and by water diverted from Mann Creek during times of high runoff or during the nonirrigation season.

Operating Agencies

Mann Creek Dam and Reservoir and all other project facilities are operated by the Mann Creek Irrigation District.

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First settlers in the area were attracted in the early 1860`s by rich bottom lands. Most of them engaged in stock raising. The creation of Olds Ferry across the Snake River and the location of three forts in the vicinity of Weiser stimulated early development. Completion of the Union Pacific Railroad through Weiser and a branch line north to New Meadows in the 1890`s gave further impetus to agricultural settlement of the area.

Early development of the project lands for irrigation was by direct diversions from Monroe and Mann Creeks. Ditches were constructed to meet the individual needs of the early farms.

The Mann Creek decree of July 17, 1919, established the water rights for the various ditches diverting from Mann Creek and the water rights for Barton Reservoir.


The Bureau of Reclamation began the investigation of Mann Creek at the Spangler Reservoir site in 1938. A storage application was made in 1940 by the United States for 10,000 acre-feet of water. Results of the investigation were presented in a report on the Mann Creek Project dated October 1940. The project was approved for construction in July 1941 under the terms of the Water Conservation and Utilization Act. The land-owners of the Mann Creek area failed to approve the proposed repayment contract by a narrow margin, and the project was not constructed. One of the principal reasons given for rejection of the proposed plan was that the landowners below the Joslyn Ditch would not receive storage water at low reservoir stages.

An alternative reservoir site about 1 mile upstream from the Spangler site that permitted diversion to the Joslyn Ditch at all times was investigated. Reauthorization of the Mann Creek Project, with development of storage at the Yoder site, was recommended with a number of other projects for construction in a basin-pooling plan under which irrigation projects would receive financial assistance from power projects in the Columbia River Basin. The proposals were not adopted by Congress.

A new study was prepared on the Spangler site in 1958, followed by authorization and development of a definite plan.


The Mann Creek Project was authorized for construction by the Act of August 16, 1962 (76 Stat. 388, Public Law 87-589).

Project purposes are irrigation, recreation, and conservation and development of fish and wildlife. The recreation purpose is associated with the minimum basic recreation facilities. The fish and wildlife purpose is related to works included for fishery mitigation.


Construction of Mann Creek Dam and Reservoir began in 1965 and was completed in 1967.

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Principal crops raised are grain, hay, pasture, and fruit.

Recreation, Fish & Wildlife

Mann Creek Reservoir lies in the rolling foothills of the Hitt Mountains in southwestern Idaho. The reservoir area encompasses a total of 936 acres, with 283 acres of water surface providing almost 5 miles of shoreline. Facilities have been constructed for camping, picnicking, and boat launching and mooring. A major north-south route for Idaho, U.S. Highway 95, passes about 1 mile from the reservoir but recreational use is light, mostly limited to picnicking and fishing, and the majority of the visitors are local residents. There is good fishing for trout at the reservoir, which is stocked annually. Migrating waterfowl use the reservoir for a resting area, and both waterfowl and upland game birds are hunted in season.

Flood Control

Before construction of Mann Creek Dam, the lands along Mann Creek were threatened each year with flood damages. Operation of Mann Creek Reservoir in regulating peak flows of Mann Creek has been helpful in flood prevention. Flood control operations are informal, but the Mann Creek Irrigation District operates the reservoir to the extent possible to reduce downstream flooding while assuring filling of the reservoir at the end of the spring runoff.

Endangered Species Act

The Pacific Northwest Region consults with the NOAA Fisheries and the US Fish and Wildlife Service to ensure that project operations and other activities do not jeopardize ESA-listed species or their critical habitats. Fish and Wildlife Service (FWS) and National Marine Fisheries Service (NMFS) provided Biological Opinions on Reclamation's Operations and Maintenance above Lower Brownlee Reservoir. The Mann Creek Project is one of the 12 projects covered in the Opinions.

If conditions don't change, these Opinions should be valid through 2035.

For more information on ESA related activities please go to:

For more information on the fish and wildlife program, please go to:  http://www.usbr.gov/pn/programs/fish_wild/index.html   

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Last updated: Jul 26, 2012