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 and riparian habitat. There are 53 fish species in the basin, none of them endemic. The Mackenzie River has a similar range of fish fauna to the Mississippi River system. It is believed that the two river systems were connected during the Ice Ages by meltwater lakes and channels, allowing fish in the two rivers to interbreed. Fish in the Mackenzie River proper include the northern pike, several minnow species, and lake whitefish. Fish in the southern half of the watershed are genetically isolated from those of the northern half due to large rapids on the Slave River preventing fish