Originating in northwestern Montana on the eastern slopes of the Rocky Mountains, the river flows northeast across the International Boundary into Alberta, emptying into the Oldman River near Lethbridge. Sharing of the waters is governed by Article VI of the Boundary Waters Treaty (1909) and the 1921 Order of the International Joint Commission.
Under these agreements Canada (Alberta) is entitled the receive the following quantities of the natural flow of the St. Mary River at its crossing of the International Boundary:
Non-irrigation season (November 1 to March 31)
50 per cent of the natural flow
Irrigation season (April 1 to October 31)
For natural flow rate up to 18.86 m³/s (666 cubic feet per second (cfs))
75 per cent of the natural flow
For natural flow rates in excess of 18.86 m³/s (666 cfs)
A prior allocation of 14.16 m³/s (500 cfs) plus 50 per cent of the natural flow
above 18.86 m³/s (666 cfs)
Compliance during the irrigation season is computed on a semi- monthly basis, with
any shortfalls due in the next balancing period. Surplus deliveries by the upstream
jurisdiction cannot be banked or used to offset future delivery shortfalls.
As water uses are relatively minor during the non-irrigation season, compliance is
currently not monitored for this period.
The irrigation and non-irrigation season natural flow volumes for the St. Mary River
at the International Boundary along with Canadian (Alberta) and U.S. entitlements
and the flow actually received by Canada can be seen by clicking the link below.
Updated: Mar 6, 2015