The Green River is a river in the states of Wyoming and Utah, in the United States. It is the largest tributary of the Colorado River, at about 730 miles (1,175 km) long. Its headwaters are in the west part of Wyoming, in a subrange of the Rocky Mountains known as the Wind River Range. From there, it flows south, then turns east for a short while, then flows south, west, and south into the Reservoir, formed by Fontenelle Dam, and Flaming Gorge Reservoir, which is formed by the Flaming Gorge Dam. 