Mackenzie River 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 and a volume of . Great Slave Lake is slightly smaller, with an area of and containing 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
Mackenzie River Lakes are remnants. Today's Mackenzie River is very young in geologic terms – its channel formed over a period of no more than several thousand years as the ice sheet retreated. Prior to the ice ages, only the Peel River tributary flowed through what is now the Mackenzie Delta into the Arctic Ocean. The other tributaries of the Mackenzie combined into the "Bell River" which flowed east into Hudson Bay. During glaciation the weight of the ice sheet depressed northern Canada's terrain to such an extent that when the ice retreated, the Mackenzie system was captured to lower elevations in