Largest river in oct. The Duncan River is a long river in the Canadian province of British Columbia. Its drainage basin is in area. It is part of the Columbia River basin, being tributary via Kootenay Lake to the Kootenay River, which is a tributary of the Columbia River. It forms part of the boundary between the Selkirk Mountains, to its west, and the Purcell Mountains, to its east (the boundary northwards is the Beaver River).