The river originates in Great Slave Lake, in the Northwest Territories, and flows north into the Arctic Ocean. It is the(longest river in Canada at 1,738 kilometres (1,080 mi) ) and, together with its headstreams the Peace and the Finlay,(the second longest river in North America at 4,241 kilometres (2,635 mi) in length). The Mackenzie and its tributaries drain 1,805,200 square kilometres (697,000 sq mi).