Assiniboine (əsĭnˈəboin) [key], river, 590 mi (950 km) long, rising in S Sask., Canada, and flowing SE into Man. then E to the Red River at Winnipeg; named for the local Native Americans, the Assiniboine. The Qu'Appelle and Souris rivers are its chief tributaries. The Assiniboine valley is one of Canada's leading wheat-growing areas. The river was explored by the Vérendrye family in 1736, and forts were built at its mouth and near the site of Portage la Prairie. Settlement spread westward along the river from the Red River valley to the plains. In 1970 the Portage Diversion was built on the river above Portage la Prairie to redirect floodwaters to Lake Manitoba.
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