Mackenzie, river, c.1,120 mi (1,800 km) long, issuing from Great Slave Lake, Northwest Territories, Canada, and flowing generally NW to the Arctic Ocean through a great delta. Between Great Slave Lake and Lake Athabasca it is known as the Slave River. At Lake Athabasca, the Finlay-Peace river system and the Athabasca River join the Mackenzie. The Finlay-Peace-Mackenzie system (c.2,600 mi/4,180 km long) is the second longest continuous stream in North America. The Liard River is the largest tributary flowing directly into the Mackenzie. The river is navigable from the Arctic Ocean to Great Slave Lake between June and October. Between Great Slave Lake and Lake Athabasca there are rapids (14 mi/23 km) that must be portaged; above the rapids are more than 400 mi (644 km) of navigable waters. The Liard River affords transportation between Fort Nelson, British Columbia, and the Arctic; the Athabasca-Mackenzie system is followed by a major shipping route between Edmonton, Alta., and the Arctic. Numerous lakes in the Mackenzie basin act as reservoirs and natural flood controls. The basin, flanked by the Rocky Mts. and the Canadian Shield, is the northern portion of the Great Plains of North America; arctic air masses follow the valley south into the interior of the continent. Much of the Mackenzie valley is heavily forested and, where climate permits, its deep soil is well suited to agriculture. Numerous trading posts were established along the Mackenzie in the early part of the 19th cent. and fur trapping is still an important activity there; the chief trading posts are Fort Simpson, Fort Providence, and Aklavik. The region was the domain of fur traders until the 1930s when vast oil fields and other mineral resources were discovered; Norman Wells is the chief oil-producing town. In the early 1970s large natural gas fields were discovered in the Mackenzie delta region. A plan to construct the Mackenzie Valley Pipeline from the Arctic Ocean to Alberta, which would have been the greatest construction project ever undertaken, was shelved in 1977 after a federal royal commission concluded that, though feasible, the project involved serious legal, political, and environmental problems. Peter Pond was possibly the first European to enter (1777) the Mackenzie drainage area, but Sir Alexander Mackenzie, the 19th-century Canadian explorer, was the first to descend (1789) the river to the Arctic Ocean.