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Lower Marias Unit
Photo of Tiber Dam Spillway, Mont.
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Pick Sloan Missouri Basin Program
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Lower Marias Unit Project History (192 KB) (pdf)
General Description| Plan| Development| Benefits
General Description

The Lower Marias Unit of the Pick-Sloan Missouri Basin Program is in north-central Montana along the Marias River. The unit has an adequate supply of irrigation water to irrigate 127,000 acres of land and also will control floods to make possible the multiple purpose use of Fort Peck Reservoir.

The original Reclamation irrigation project features were not constructed because the irrigation district did not negotiate a repayment contract with the United States and those features are no longer part of the Lower Marias Unit. 

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Since completion in 1956, Tiber Dam and Lake Elwell have served important functions in flood control, recreation, fish and wildlife, and municipal and industrial water supplies.

A minimum flow will be maintained in the river below Lake Elwell to meet downstream demands, such as stock water and support of fish and wildlife, and other downstream water rights.

The 400,838-acre-foot capacity to be allocated exclusively to flood control in Lake Elwell will regulate the Marias River and assist in flood control of the Missouri River. Sufficient dead storage will be available in Lake Elwell to impound the entire silt load of the river for several hundred years.

Municipal and domestic water from Lake Elwell is being contracted for by the city of Chester, the Tiber County Water District, and Devon Water Inc.

Facility Descriptions
Tiber Dam

Tiber Dam is a zoned earthfill structure containing 9.8 million cubic yards of embankment materials. The dam has a crest length of 4,300 feet, and a structural height of 211 feet. A 14-foot-diameter, concrete-lined tunnel, which was used for river diversion during construction, serves as a river outlet. An earthfill dike closes a saddle on the south rim of the reservoir near the dam. This dike contains 1.95 million cubic yards of embankment, a 16,650 feet long, and has a maximum height of 65 feet. The reservoir, Lake Elwell, has a storage capacity of 1,368,157 acre-feet; 389,695 acre-feet of this capacity is allocated to active conservation and 400,838 acre-feet to exclusive flood control.

The river outlet works are controlled by a 5.0-by 5.0-foot, high-pressure gate having a capacity of 1,540 cubic feet per second, and discharging through a 72-inch-diameter steel pipe. Constructed as part of the safety of dams modification, the auxiliary outlet works, at the left abutment of the dam, are controlled by a 7.25- by 9.25 foot outlet gate with a capacity of 4,240 cubic feet per second, and discharging into a 10.75-foot-diameter, concrete-lined tunnel.

Settlement of the Tiber Dam spillway crest section began following initial filling of the reservoir in 1956. Restrictions were placed on reservoir operating levels in the late 1950`s to safeguard the structure until repairs could be made. The settling continued despite attempts to halt it. The rate of settlement was alarming following the flood of 1964 and the heavy runoff of 1965. This settlement was attributed to a weakness of the underlying shale formation in which small lenses of gypsum were slowly being dissolved as water passed through the shale. Measures to protect the structure were approved by Congress and construction initiated in 1967. This work, completed in 1970, consisted of modifying the canal outlet works for use as an auxiliary outlet works and closing the entrance channel of the spillway by a temporary earthfill cofferdam. To accommodate these changed conditions, the reservoir operating criteria was further revised and the active capacity was eliminated. Work on modification of the spillway and to restore active conservation capacity began in 1976. This work, completed in October 1981, consisted of replacing the upstream section of the spillway and raising the dam 5 feet.

Beginning in 1982, and through 1983, Lake Elwell was filled to its original design capacity at an elevation of 2,993.0 ft. Normal reservoir operations have since been restored.

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In 1846. the American Fur Company established headquarters on the site which is now Fort Benton, about 35 miles southeast of Tiber Dam. It served as a terminus of river traffic from St. Louis, Missouri, during the 1860`s and 1870`s. Stock raising began in the 1880`s and large cattle companies predominated until the drought of 1903. Public lands suitable for agriculture were homesteaded in tracts of 160 to 320 acres between 1908 and 1914. From 1917 to 1920, drought prevailed and many farms were abandoned. This gradually resulted in increased size of farm operations until the average dryland farm unit of today consists of more than 1,600 acres.


The first investigation of the area which is now within the Lower Marias Unit was started in 1903 by the Reclamation Service. The surveys disclosed that a potential irrigation project could use the Marias River water for irrigation of lands lying between that river and Big Sandy Creek. The plan centered on a damsite near the Liberty Hill County line.

Investigations and studies of the irrigation potential were numerous, but no action was taken on these early proposals until in 1930 when local interest reached a new high because of the drought. The Montana State Water Conservation Board reviewed the previous studies and issued a report in 1935. In 1937, a committee was formed to promote development of Marias water resources. As a result of the committee`s action, an exhaustive survey of the project possibilities was made by the Bureau of Reclamation and a report was released in 1939.

In further investigations, which covered the entire river system, all possible damsites were compared and pumping projects were considered. The Lower Marias Unit with Tiber Dam and Lake Elwell was selected as the most desirable plan of development because of the relative need of the local area and its important relationship to the overall Missouri River Basin Program (now Pick-Sloan Missouri Basin Program). During 1940-1941, the recorded flow of the Marias River reached an all time low, limiting to 110,000 acres the land that could be irrigated by initial storage of the Marias River runoff. The studies were completed in 1941 and the plan selected for the Lower Marias Unit was presented in an unpublished report dated April 1944.


The unit was authorized by the Flood Control Act of December 22, 1944, Public Law 534, which approved the general comprehensive plan set forth in Senate Document 191 and House Document 475, as revised and coordinated by Senate Document 247, 78th Congress, 2d session. Additional appropriations were approved August 10, 1972, by Public Law 92-371, 86 Stat. 525.


Construction of Tiber Dam began September 15, 1952, and was completed March 6, 1956. The final river closure was made on October 27, 1954. Construction of the auxiliary outlet works and spillway cofferdam was begun in December 1967 and completed in June 1969. Construction on the spillway modification was begun in December 1976.

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Five private irrigators and the Tiber Irrigation Co., which consists of 17 individuals, are pumping water from Lake Elwell under long-term water service contracts. The maximum amount of water diverted per year is about 6,000 acre-feet. The total acreage irrigated in 1982 was about 3,000 acres.


Lake Elwell has been planted with rainbow trout. Picnicking, swimming, boating, fishing, and hunting are popular sports, and public-use areas with minimum facilities are available.

Recreation.gov has more specific information about recreational opportunities at Lake Elwell.

Flood Control

In addition to contributing to the abatement of floods on the Marias River, Lake Elwell makes possible greater utilization of Fort Peck Reservoir on the Missouri River and provides regulation of the Marias River to meet downstream demands for fish and wildlife, irrigation, and silt control. Lake Elwell has a flood control capacity of 400,838 acre-feet, including 301,484 acre-feet of replacement water for Fort Peck utilization, and a surcharge capacity of 187,740 acre-feet for a total flood capacity of 588,578 acre feet and, since construction of the dam, has prevented $54.7 million in flood damages.

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Last updated: May 16, 2012