River system, Northwest Territories, Canada. It flows northward from Great Slave Lake into the Beaufort Sea of the Arctic Ocean. Its basin, with an area of 697,000 sq mi (1,805,200 sq km), is the largest in Canada. It is 1,025 mi (1,650 km) long and 1–2 mi (1.5–3 km) wide. With the Finlay River, its farthest headstream, the entire system is 2,635 mi (4,241 km) long, making it the second longest river in North America. It was discovered by the explorer Alexander Mackenzie in 1789.
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