The St. Mary River Siphon consists of two 90- inch riveted steel barrels that traverse the valley from the inlet, transition to an 84-inch diameter at the river crossing, transition back to 90 inches in diameter and traverse up the valley slope to the outlet. The barrels are approximately 3,200 feet in length. The discharge of each barrel is approximately 425 cfs at a velocity of 9.63 feet per second in the 90-inch section and 11.05 feet per second in the 84-inch section. The maximum head on the Siphon is 165 feet.

The downstream barrel was constructed from 1912-15, and the upstream barrel was constructed in 1925-26. The downstream barrel is buried for approximately half its length at a depth of 3-5 feet, and the right barrel runs entirely above ground. The left barrel has undergone a series of extensive repairs due to problems associated with being buried, such as seepage, corrosion, and buckling. A cathodic protection system was installed in the 1950s. Unstable valley sidewalls have resulted in the downslope movement of the steel barrels and concrete supports producing buckling in the siphon barrels, and compression of the expansion/contraction joints.