Milk River Project (St. Mary Canal) and St. Mary Siphon Blowout

Diversion Dam: (Satellite, this is the headwaters of the canal.

Kennedy Siphon: (Satellite)

St. Mary Siphon: (Satellite)

Halls Coulee Siphon: (Satellite)

Spillway into Milk River: (Satellite)

The St. Mary Canal takes water from the St. Mary River before it enters Canada and moves the water east to dump into the Milk River. This supplies water for towns and agriculture in north-central Montana.

ktvq
This canal is evidently also called Montana's Hi-Line and 18,000 residents depend on it for water in the 200-mile Milk River Irrigation Project.
Congress appropriated money in 1905 to build the system. But, after it was built, Congress changed their mind and quit funding maintenance.

On Jun 17, 2024 the St. Mary Siphon blew out.

ktvq, Credit: Milk River Joint Board of Control

The blowout not only stopped the source of water to the irrigation project, it caused flooding in the St. Mary river.

MontanaFreePress, Credit: Bureau of Reclamation
"The failure involves the St. Mary River Siphon, which is composed of a pair of 90-inch [2.3m] riveted steel barrels that traverse a 3,200-foot [975m] section of the St. Mary Valley to divert water to the North Fork of the Milk River." The failure occurred "while U.S. Bureau of Reclamation personnel were monitoring a crack in the siphon."
The downstream pipes were built in 1915. Five hours after one barrel broke, the other one also broke. And the concrete structures holding the pipes were washed away.

krtv_repair, Credit: MTN News

krtv_repair

same video

krtv_repair, Credit: MTN News

usbr
[I wonder where the second pipe broke.]
"The siphon has a maximum head of 165 feet and was constructed in two phases, with the downstream barrel completed between 1912-15 and the upstream barrel after 1925. The siphon has undergone extensive repairs due to seepage, corrosion, and buckling. A cathodic protection system was installed in the 1950s to address these problems. However, unstable valley sidewalls have caused further movement of the steel barrels and concrete supports, leading to additional damage."

Sun 25, 2025:

krtv_restored
"A $70 million project has successfully restored water flow to the Milk River along Montana's Hi-Line"

A video on the krtv_restored webpage
[I quit watching when they started talking about every agency stepped up.]

same video
[I took this screenshot because of the view of the diversion dam release structure.]

And this is the other end of the canal. This structure had a catastrophic failure on May 17, 2020. They replaced Drop 2 as well as Drop 5. [MilkRiverProject_emergency]

The Milk River Project posted

3:06 video
Water was restored on Oct 10, 2020

The Milk River Project posted

🎥 As Promised — Last Night’s Flyover! 🎥

Just after 10:00 p.m. last night, the St. Mary Siphon outlet finally came to life as water completely filled the pipe and began its journey down the historic St. Mary Canal. This drone footage flies from the outlet structure to the inlet structure of the St. Mary Siphon — a perspective only possible thanks to the skill, dedication, and long hours put in by these amazing crew.

💧 We’re officially flowing — and it’s beautiful.

Quick update: the water is now about 1 miles from the Halls Coulee Siphon inlet and we expect it to arrive at appx 5:30 p.m.

Fun fact: it's currently moving at about 0.43 mph overall — slow and steady wins the race, right?

save video, the inlet structure.

I guess this is also the intake structure.

MontanaFreePress_expected, Credit: Milk river Project

The old inlet structure had no provision for stop logs or control gates. It looks like they learned their lesson and the new inlet structure can stop the flow of water into the pipes.

Satellite

Another view of the inlet structure as the water first approached it.

Facebook Reel

And another view of the diversion dam after they do the first release into the repaired canal.

Facebook Reel

The Milk River Project posted four photos and two videos with the comment:

💧 Halls Coulee Update: Why We’re Starting Early

Let’s talk about Halls Coulee and the difficult—but necessary—decision to stop diversions to the Milk River around August 15th so crews can finally address this aging structure.

Over the off-season, teams worked hard to temporarily stabilize the siphon by installing Hydratite seals in the worst trouble spots. These are holding for now, but this siphon is over 100 years old, and every patch just shifts the problem elsewhere as the pipe expands and contracts.

✅ A bypass channel is now in place, allowing NW Construction and Sletten Construction to begin dirtwork and start building the outlet structure while water continues to flow.

The inlet sits on an extremely steep slope with wide wing walls, and safely setting the new structure alongside the failing one within our right-of-way would require extensive shoring designs that come with serious safety and engineering challenges.

With the diversion running until August 15th, we estimate approximately 37,000 acre-feet of storage by Sept 30 will be in Fresno Reservoir to help meet municipal demands throughout the winter.

We are also working through one final permit for a land use agreement outside of our right-of-way (for parking, staging, road access, material sales, etc.) with the Blackfeet Tribe, and once that’s in place, all permits will be secured.

Starting early not only reduces risk to future water deliveries and worker safety but also helps us avoid heavy winter construction—think heated tents, blankets, and costly delays in extreme cold.

We understand this timeline isn’t ideal for everyone, and it’s not a decision we made lightly. But as a team, we believe moving forward now is the safest and most responsible choice to protect the Milk River Basin’s water supply for decades to come.

[The two videos were of active leaks.]

Matt Bolland: With the old system being above ground, leaks were easy to spot, like these videos. With the new pipes underground, how difficult will it be to detect minor to major leaks before they become catastrophic?

The Milk River Project: Matt Bolland each of the St. Mary and Halls Coulee pipes had sections that were buried. From previous experience, water is incredibly powerful—when there’s an issue, you’ll either see it leaking or notice the vegetation above starting to change.

Jody Mann: How much water does Canada take that is diverted by the St. Mary project? How is this monitored?

The Milk River Project: Jody Mann Right now, Canada will not take any irrigation water from the St. Mary Project—but our water volumes will help their municipalities tremendously. Under the Boundary Waters Treaty of 1909 and subsequent agreements, Canada is entitled to receive up to half of the natural flow of the Milk River, but none of the water diverted through the St. Mary Canal is allocated for Canadian irrigation. There is also deficit accounting but we haven’t run one this year either with the failure. Little complicated.

The volume delivered is measured using USGS gaging stations at the International Boundary, which record and report real-time flows. The International Joint Commission through Accredited Officers oversees compliance and reviews these records to ensure both countries are meeting treaty obligations and accurately accounting for any deliveries.

1

2

3

4

Tcrowe For MT commented on the above post
Got to see the historic Hall’s Coulee siphon last week during the tour.

The Milk River Project has posted weekly updates with lots of nice photos. One update indicated that the diversion dam was also rebuilt during the St. Mary Siphon caused outage.