Severn (sĕvˈərn) [key]. 1 River, c.420 mi (680 km) long, rising in W Ont., Canada, and flowing NE through Severn Lake to Hudson Bay. Fort Severn, a Hudson's Bay Company trading post established (1689) at the mouth of the river, was captured (1690) by Pierre le Moyne, sieur d'Iberville. The post was rebuilt in 1759 and has been in continuous operation since. 2 River, c.20 mi (30 km) long, rising from the north end of Lake Couchiching, S Ont., Canada, and flowing NW to Georgian Bay of Lake Huron. It drains Lake Couchiching and Lake Simcoe and forms part of the inland waterway system linking Georgian Bay with Lake Ontario via the Trent Canal. There are two large hydroelectric stations on its course.