Water diversions have started at St. Mary Diversion Dam

BILLINGS, Mont. - The Bureau of Reclamation announced that construction at the Halls Coulee Siphon has finished ahead of schedule, marking another major milestone in the ongoing effort to restore and modernize critical St. Mary Project infrastructure. Water diversions at the St. Mary Diversion Dam began Tuesday, April 7.

The St. Mary system is a vital component of Reclamation's Milk River Project in north-central Montana. Water delivered through the St. Mary facilities supports approximately 110,000 acres of irrigated agriculture, approximately 150 individual pumpers, several communities, and the Bowdoin National Wildlife Refuge. In an average year, the St. Mary system provides more than 60 percent of the Milk River Project water supply, and in dry years that share can exceed 80 percent.

"This milestone shows what can be accomplished when partners come together around a shared purpose," said Ryan Newman, Bureau of Reclamation Montana Area Office manager. "With the Halls Coulee Siphon complete and diversions resuming at St. Mary, we are restoring a system that is essential to farms, communities, and water users across north-central Montana. It took strong coordination, persistence, and commitment from many partners to reach this point."

The announcement follows extensive work to address aging infrastructure within the St. Mary Unit. The St. Mary Diversion Dam and Canal were originally completed in 1915 as part of the Milk River Project. On June 17, 2024, both barrels of the St. Mary River siphon experienced structural failure, forcing an immediate stop to diversions from the St. Mary River into the Milk River system. Reclamation then made emergency determinations that allowed design, environmental review, and construction activities to move forward on an accelerated schedule for both the St. Mary siphon and Halls Coulee Siphon.

Reclamation completed replacement of the St. Mary siphon in June 2025, restoring a major section of the system and allowing crews to advance work on Halls Coulee. The completion of Halls Coulee now represents another important step in improving reliability across the St. Mary Canal system and supporting continued delivery of water to project beneficiaries.

The broader St. Mary Diversion Dam Replacement Project also continues near Babb, Montana. That work includes replacement of aging diversion and headworks features with a modernized system designed to improve operations, enhance reliability, and provide fish passage and fish protection measures for bull trout in compliance with the Endangered Species Act. Reclamation has stated that the larger diversion dam replacement work began on site in 2024 and is anticipated to continue through 2028.

Reclamation will continue coordinating closely with project partners and stakeholders as work progresses across the St. Mary system.

For more information, visit Reclamation's Montana Area Office website at, https://www.usbr.gov/gp/mtao/.

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