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 from swimming upstream. Migratory birds use the three major deltas in the Mackenzie River basin – the Mackenzie Delta and the inland Slave and Peace-Athabasca Deltas – as resting and breeding areas. The latter is located at the convergence of four major North American migratory routes, or flyways. As recently as the mid-twentieth century, more than 400,000 birds passed through during the spring and up to a million in autumn. Some 215 bird species in total have been catalogued in the delta, including species such as the whooping crane, peregrine falcon and bald eagle. The construction of the W.A.C. Bennett Dam on the Peace River has reduced the seasonal variations of water levels in the delta, causing damage to its ecosystem. Bird populations have seen a steady decline since the 1960s. Water mammals such as beavers and muskrats are extremely common in the Mackenzie Delta and surrounding areas of muskeg. The Mackenzie estuary is also a calving area for beluga whales. History The Mackenzie valley is believed to have been the path taken by prehistoric peoples during the initial human migration from Asia to North America more than 10,000 years ago. However, archaeological evidence of human habitation along the Mackenzie is scant, despite the efforts of many researchers. Many archaeological sites have probably been destroyed by flooding, freeze-thaw and erosion. The Inuvialuit, Gwich'in and other Indigenous peoples have lived along the river for thousands of years; however, the oldest evidence of continuous occupation stretches back only about 1300–1400 years, at the Gwich'in community of Tsiigehtchic. The Mackenzie provided the major route into Canada's northern interior for European explorers as early as the late 18th century. Scottish explorer Alexander Mackenzie travelled the river in the hope it would lead to the Pacific Ocean, but instead reached its mouth on the Arctic Ocean on 14 July 1789.