The St. Mary's Diversion Works, located on the St. Mary River 0.75 downstream from Lower St. Mary Lake, is a 6-foot-high concrete weir and sluiceway with a length of 198 feet and a total volume of 1,200 cubic yards.
The St. Mary Canal begins at St. Mary Diversion Dam on the west side of St. Mary River and crosses the river 9.5 miles below the diversion through a two-barrel steel-plate siphon 90 inches in diameter and 3,600 feet in length. Eight miles below the St. Mary crossing a second two-barrel steel-plate siphon, 78 inches in diameter and 1,405 feet long, conveys the water across Hall's Coulee. A series of five large concrete drops at the lower end of the 29-mile canal provide a total fall of 214 feet to the point where the water is discharged into North Fork Milk River. Design capacity of the canal is 850 cubic feet per second.