Athabasca (ăthəbăsˈkə) [key], river, 765 mi (1,231 km) long, rising in the Columbia snowfield of the Canadian Rockies near the Alta.–British Columbia line and flowing N through Jasper National Park, then NE and N across central Alta. to Lake Athabasca. It is the southernmost headstream of the Mackenzie River. Its chief tributaries are the Pembina, Lesser Slave, and Clearwater rivers. The Athabasca River has long been the main route to the Mackenzie valley. There are extensive deposits of oil-bearing sand along the river near Fort McMurray.