Mackenzie River - Watershed

Watershed

At 1,805,200 square kilometres (697,000 sq mi), the Mackenzie River's watershed or drainage basin is the largest in Canada, encompassing nearly 20% of the country. From its farthest headwaters at Thutade Lake in the Omineca Mountains to its mouth, the Mackenzie stretches for 4,241 km (2,635 mi) across western Canada, making it the longest river system in the nation and the thirteenth longest in the world. The river discharges more than 325 cubic kilometres (78 cu mi) of water each year, accounting for roughly 11% of the total river flow into the Arctic Ocean. The Mackenzie's outflow holds a major role in the local climate above the Arctic Ocean with large amounts of warmer fresh water mixing with the cold seawater.

Many major watersheds of North America border on the drainage of the Mackenzie River. 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 and its headstreams the Pelly and Stewart rivers, which flow to the Bering Strait; and the Fraser River and Columbia River systems, both of which run to the Pacific Ocean. Lowland divides in the north distinguish the Mackenzie basin from those of the Anderson, Horton, Coppermine and Back Rivers – all of which empty into the Arctic. Eastern watersheds bordering on that of the Mackenzie include those of the Thelon and Churchill Rivers, both of which flow into Hudson Bay. On the south, the Mackenzie watershed borders that of the North Saskatchewan River, part of the Nelson River system, which empties into Hudson Bay after draining much of south-central Canada.

Through its many tributaries, the Mackenzie River basin covers portions of five Canadian provinces and territories – British Columbia (BC), Alberta, Saskatchewan, and the Yukon and Northwest Territories. The two largest headwaters forks, the Peace and Athabasca Rivers, drain much of the central Alberta prairie and the Rocky Mountains in northern BC then combine into the Slave River at the Peace-Athabasca Delta near Lake Athabasca, which also receives runoff from northwestern Saskatchewan. The Slave is the primary feeder of Great Slave Lake (contributing about 77% of the water); other inflows include the Taltson, Lockhart and Hay Rivers, the latter of which also extends into Alberta and BC. Direct tributaries of the Mackenzie from the west such as the Liard and Peel Rivers carry runoff from the mountains of the eastern Yukon.

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 km2 (12,028 sq mi) and a volume of 2,236 km3 (536 cu mi). Great Slave Lake is slightly smaller, with an area of 28,568 km2 (11,030 sq mi) and containing 2,088 km3 (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 km2 (3,000 sq mi). Six other lakes in the watershed cover more than 1,000 km2 (390 sq mi), including the Williston Lake reservoir, the second-largest artificial lake in North America, on the Peace River.

With an average annual flow of 9,910 m3/s (350,000 cu ft/s), the Mackenzie River has the highest discharge of any river in Canada and is the fourteenth largest in the world in this respect. About 60% of the water comes from the western half of the basin, which includes the Rocky, Selwyn, and Mackenzie mountain ranges out of which spring major tributaries such as the Peace and Liard Rivers, which contribute 23% and 27% of the total flow, respectively. In contrast the eastern half, despite being dominated by marshland and large lakes, provides only about 25% of the Mackenzie's discharge. During peak flow in the spring, the difference in discharge between the two halves of the watershed becomes even more marked. While large amounts of snow and glacial melt dramatically drive up water levels in the Mackenzie's western tributaries, large lakes in the eastern basin retard springtime discharges. Breakup of ice jams caused by sudden rises in temperature – a phenomenon especially pronounced on the Mackenzie – further exacerbate flood peaks. In full flood, the Peace River can carry so much water that it inundates its delta and backs upstream into Lake Athabasca, and the excess water can only flow out after the Peace has receded.