Slave, river, c.310 mi (500 km) long, Northwest Territories, Canada. It comprises the middle sections of the Mackenzie River system. The river channels the waters of Lake Athabasca and the Peace River into Great Slave Lake at Fort Resolution. It is navigable for steamers except for the rapids between Fort Fitzgerald and Fort Smith, where it breaks through the Cariboo Hills. There is a wagon road portage (16 mi/26 km long) around the rapids.