The St. Mary River is a north-flowing tributary to the Oldman River in Alberta, itself a tributary of the South Saskatchewan River. The Saint Mary together with the Belly River and Waterton River drain a small portion of Montana, in the United States, to the Hudson Bay watershed in Canada. The river rises as a stream on Gunsight Mountain in Glacier National Park and flows into Gunsight Lake, then into Saint Mary Lake, exits the park and flows on into Lower St. Mary Lake in the Blackfeet Indian Reservation. The outlet at the north end of Lower St. Mary Lake is south of Babb. From the reservation, the St. Mary River flows north into Alberta and the St. Mary Reservoir. It flows into the Oldman River which eventually reaches the Saskatchewan River.