The St. Regis River is an approximately 32.5-mile (52.3 km) tributary of the Clark Fork of the Columbia River in Montana in the USA. It begins at Lookout Pass on the Idaho/Montana border, flows east and ends 1.2 miles (1.9 km) west of St. Regis, Montana at the confluence with the Clark Fork. Its drainage is 330 square miles (850 km²). The record high flow for the St.