High Quality Content by WIKIPEDIA articles! The Mackenzie River (Slavey language: Deh-Cho, big river) is the largest and longest river system in Canada. It flows through a vast, isolated region of forest and tundra entirely within the country`s Northwest Territories, although its many tributaries reach into four other Canadian provinces and territories. The river`s mainstem runs 1,738 kilometres (1,080 mi) in a northerly direction to the Arctic Ocean, draining a vast area nearly the size of Indonesia. It is the largest river flowing into the Arctic from North America, and with its tributaries is one of the longest rivers in the world.