Flaming Gorge Reservoir is the largest reservoir in Wyoming, on the Green River, impounded behind the Flaming Gorge Dam. Construction on the dam began in 1958 and was completed in 1964. The reservoir stores 3,788,900 acre-feet (4.6735×109 m3) of water when measured at an elevation of 6,040 feet (1,841 m) above sea-level (maximum).

The Green River, located in the western United States, is the chief tributary of the Colorado River. The watershed of the river, known as the Green River Basin, covers parts of the U.S. states of Wyoming, Utah, and Colorado. The Green River is 730 miles (1,170 km) long, beginning in the Wind River Mountains of Wyoming and flowing through Wyoming and Utah for most of its course, except for a short segment of 40 miles (64 km) in western Colorado. Much of the route traverses the arid Colorado Plateau, where the river has carved some of the most spectacular canyons in the United States. The Green is slightly smaller than Colorado when the two rivers merge but typically carries a larger load of silt. The average yearly mean flow of the river at Green River, Utah is 6,121 cubic feet (173.3 m3) per second.