US$71 million Fresno Dam repairs proposed in Montana

The Bureau of Reclamation has proposed the Fresno Dam in Montana for repairs and modifications worth US$71 million after it was discovered that the dam has sunk about 10 feet since construction was completed in 1939. The dam, located 14 miles Northwest of Havre, is a 10-foot-tall, earth-filled reservoir that stretches more than a half-mile to impound the Milk River. The crest of the Fresno Dam is paved and serves as a public route between State Highway 232 and U.S. Highway 2, with a steel bridge providing access across the dam spillway. Since it was built, the reservoir has shifted as the sandstone, clay and shale beneath the structure has been compressed. Early efforts to address the problem resulted in the crest of the dam being raised to meet the design’s required elevation.

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It was built to provide irrigation water to about 140,000 acres of land across eight irrigation districts, as well as to control floodwaters and for some municipal use. Eight years ago, the Bureau of Reclamation began analyzing the dam for structural problems, and one year ago, a Corrective Action Study was completed to “reduce the risk of dam failure.” This resulted in the dam being designated an “urgent priority” with “very high risks or likelihoods of failure”, however, not posing imminent danger.

The construction work that has been proposed to avoid a dam failure and correct the issues includes installing a vertical sand filter trench at the bottom and constructing a “state-of-the-practice embankment overlay” on the downstream slope of the dam. The work would be completed in approximately 3 years and would require the reservoir’s elevation to be 800 meters to allow for excavation on the spillway during the first year. Drawing the reservoir down is, unfortunately, likely to result in a loss of 3,200 acre-feet of irrigation water in the second year of work, about US$79,800 in lost irrigation benefits.