Mackenzie River, seasonal tributary of the Fitzroy River, eastern Queensland, Australia. Formed by the junction of the Comet and Nogoa rivers, which rise in the Eastern Highlands, it flows for 170 miles (275 km) past Comet, northeast across the Expedition Range, and then southeast, joining the Dawson River to form the Fitzroy River. Its principal tributary is the Isaac. Explored in 1844 by Ludwig Leichhardt, it was named after Sir Evan Mackenzie, a Queensland settler. Its valley supports dairy and beef cattle.