structural:expansion
Drainage basin

At 1,805,000 km (697,000 sq mi), the Mackenzie River drainage basin encompasses nearly 20 percent of Canada. About 980,000 km (380,000 sq mi), or 54 percent of the basin, lies above Great Slave Lake. Permafrost underlies about three-quarters of the watershed, reaching up to 100 m (330 ft) deep in the Mackenzie Delta. As a whole, the Mackenzie basin receives only meager to moderate rainfall, averaging 410 millimetres (16 in) over the entire basin, though mountain areas experience much higher precipitation, and areas near and north of the Arctic Circle receive much lower precipitation.

The Mackenzie drainage basin is bordered by multiple major North American watersheds. Much of the western edge of the Mackenzie basin runs along the Continental Divide. The divide separates the Mackenzie watershed from that of the Yukon River, which flows to the Bering Strait; and the Fraser River and Columbia River systems, both of which empty into the Pacific Ocean. Lowland divides in the north separate the Mackenzie basin from the Arctic Ocean watersheds of the Anderson, Horton, Coppermine and Back Rivers. In the east, the Mackenzie borders on the Hudson Bay watersheds of the Thelon and Churchill Rivers, and to the south it is bordered by the Saskatchewan – Nelson River system, which also empties into Hudson Bay. The Mackenzie system is hydrologically connected to the Hudson Bay watershed via Wollaston Lake, which is not only the source of the Fond du Lac tributary of Lake Athabasca, but also of the Cochrane River, which flows east into the Churchill River.

The eastern portion of the Mackenzie basin is dominated by vast reaches of lake-studded boreal forest and includes many of the largest lakes in North America. By both volume and surface area, Great Bear Lake is the biggest in the watershed and third largest on the continent, with a surface area of 31,153 km (12,028 sq mi) and a volume of 2,236 km (536 cu mi). Great Slave Lake is slightly smaller, with an area of 28,568 km (11,030 sq mi) and containing 2,088 km (501 cu mi) of water, although it is significantly deeper than Great Bear. The third major lake, Athabasca, is less than a third that size with an area of 7,800 km (3,000 sq mi). Six other lakes in the watershed cover more than 1,000 km (390 sq mi), including the Williston Lake reservoir, the second-largest artificial lake in North America, on the Peace River.