The Green River is a tributary of the Colorado River, 730 miles (1,170 km) long, in the western United States. The Green River Basin covers parts of Wyoming, Utah, and Colorado. The river begins in the Wind River Mountains of Wyoming, and flows through Utah for much of its course, draining the northeastern portion of the state while looping for 40 miles (64 km) into western Colorado. Much of its route is through the Colorado Plateau, some of the most spectacular canyons in the United States. It is the largest tributary of the Colorado.
South of the Flaming Gorge Dam it flows eastward, looping around the eastern tip of the Uinta Mountains going from Utah into northwestern Colorado, then south into Dinosaur National Monument and is joined by the Yampa River at Steamboat Rock. It turns westward back into Utah along the southern edge of the Uintas in Whirlpool Canyon. In Utah it meanders southwest across the Yampa Plateau and through the Uintah and Ouray Indian Reservation and the Ouray National Wildlife Refuge. Two miles south of Ouray, Utah, it is joined by Duchesne River, and three miles downstream by the White River. Ten miles farther downstream it is joined by the Willow River.
South of the plateau, it is joined by the Nine Mile River, then enters the Roan Cliffs where it flows south through the back-to-back Desolation and Gray Canyons, with a combined length of 120 miles (190 km). In Gray Canyon, it is joined by the Price River. South of the canyon it passes the town of Green River, Utah and is joined by the San Rafael River in southern Emery County. In eastern Wayne County it meanders through Canyonlands National Park where it joins the Colorado.