The *Green River was known to the Shoshone Indians as the Seeds-kee-dee-Agie, or Prairie Hen River. This name, in one version or another, was later adopted and widely used by the mountain men. Dominguez and Escalante named the Green the Rio de San Buenaventura, but the river was known by later Spaniard and Mexican explorers as the Rio Verde, or Green River. This connection with the Spanish led to the Green being known for a time as the Spanish River, but by the time Ashley floated the Green in 1825, the name "Green River" was in common use. Accounts vary as to why the river is called the Green. One has it that it is because of the color of the water; another that it is named for a member of Ashley's original party of mountain men. John C. Frémont thought that the name came from the vegetation along the banks. No one account is authoritative.
The Green River is Utah's major stream. Its beginnings are in Wyoming, on the eastern slopes of the Wind River Mountains, and it makes a forty-mile loop through northwestern Colorado, but the majority of the course of the Green lies in Utah. The river is 730 miles long; approximately 450 miles of it are in Utah. The Green drains the entire northeast corner of Utah, or about one-quarter of the entire area of the state. The landforms drained by the Green in Utah range from the highest part of the state, in the Uinta Mountains, to some of the lowest, in the Uinta Basin. In its course through Utah, the Green drops from an elevation of approximately 6,000 feet above sea level at Flaming Gorge Reservoir to about 3,000 feet at its confluence with the Colorado.