Great Bear Lake, largest lake of Canada and fourth largest of North America, c.12,275 sq mi (31,800 sq km), c.190 mi (310 km) long and from 25 to 110 mi (40–177 km) wide, Northwest Territories, on the edge of the Canadian Shield. It is drained to the W by the Great Bear River (c.100 mi/160 km long), which flows into the Mackenzie River. Even though it is one of North America's deepest (1,356 ft/413 m) lakes, its waters are open only about four months each year. The lake was explored (c.1800) by traders of the North West Company, and a trading post was later established there. Déline (formerly Fort Franklin), on the southwest shore, was built by Sir John Franklin, a British explorer, in 1825. Discoveries of rich radium ores, which are now exhausted, on the eastern side of the lake in 1930 caused much mining activity in the years immediately following; the Eldorado Mines, at Port Radium, were located there.