Assiniboine River, a river in Canada, about 590 miles (950 km) long. From its source in southeastern Saskatchewan the Assiniboine flows generally southeastward into Manitoba. Here it turns eastward and winds its way to the Red River of the North at Winnipeg. The Assiniboine's chief tributaries are the Qu'Appelle and Souris rivers. The Assiniboine Valley is one of Canada's leading wheat-producing areas.