The status of the Green River as a tributary of the Colorado River came about for mainly political reasons. Before 1921 the Colorado River began at its confluence with the Green River. Above the confluence the Colorado was called the Grand River. Colorado U.S. Representative Edward T. Taylor petitioned the Congressional Committee on Interstate and Foreign Commerce to rename the Grand River as the Colorado River. On July 25, 1921 the name change was made official in House Joint Resolution 460 of the 66th Congress, over the objections of representatives from Wyoming and Utah and the United States Geological Survey which noted that the drainage basin of the Green River was more than 70% more extensive than that of the Grand River, although the Grand carried a slightly higher volume of water at its confluence with the Green.
It rises in western Wyoming, in northern Sublette County, on the western side of the continental divide in the Bridger-Teton National Forest in the Wind River Range. It flows south through Sublette County and western Wyoming in an area known as the Upper Green River Valley, then southwest and is joined by the Big Sandy River in western Sweetwater County. At the town of La Barge, it flows into Fontanelle Reservoir, formed by Fontanelle Dam. Below there, it flows through open sage covered rolling prairie where it is crossed by the Oregon, California and Mormon emigration trails and then further south until it flows past the town of Green River and into the Flaming Gorge Reservoir in Southwestern Wyoming, formed by the Flaming Gorge Dam in northeastern Utah.
South of the dam it flows eastward, looping around the eastern tip of the Uinta Mountains going from Utah into northwestern Colorado and through Browns Park before turning west and then south into Dinosaur National Monument where it passes through the Canyon of the Lodore (Otherwise known as the Gates of Lodore) 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 (5 km) 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 mi (192 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.
The Flaming Gorge Dam in Utah is a significant regional source of water for irrigation and mining, as well as for hydroelectric power. Begun in the 1950s and finished in 1963, it was highly controversial and opposed by conservationists. Originally, a dam was to be built in Whirlpool Canyon, but the conservationist movement traded the Flaming Gorge dam for halting that proposal. Apocryphally, the Sierra Club, a not-for-profit environmental organization, lost its tax-exempt status for opposing the proposed dam.
The Green is a large, deep, powerful river. It ranges from 100 to 300 feet (30 to 100 m) wide in the upper course to 300 to wide in its lower course and ranges from 3 to 50 feet (1 to 15 m) in depth. It is navigable by small craft throughout its course and by large motorboats upstream to Flaming Gorge Dam.
Archaeological evidence indicates that the tributary canyons and sheltered areas in the river valley were home to the Fremont Culture, which flourished from the 7th century to the 13th century. The Fremont were a semi-nomadic people who lived in pithouses and are best known for the rock art on canyon walls and in sheltered overhangs.
In later centuries, the river basin was home to the Shoshone and Ute peoples, both nomadic hunters. The Shoshone inhabited the river valley north of the Uinta Mountains, whereas the Utes lived to the south. The current reservation of the Utes is in the Uinta Basin. The Shoshone called the river the Seeds-kee-dee-Agie, meaning "Prairie Hen River."
In 1776, the Spanish friars Silvestre Vélez de Escalante and Francisco Atanasio Domínguez crossed the river, naming it the Rio de San Buenaventura. The map-maker of the expedition, Captain Bernardo Miera y Pacheco, erroneously indicated that the river drained the Great Salt Lake into the Pacific Ocean. Later Spanish and Mexican explorers adopted the Rio Verde, meaning "Green River" in Spanish. The origin of the name "Green" is obscure but perhaps is based on the color of the water. The Old Spanish Trail from New Mexico to California crossed the river just above the present-day town of Green River, Utah.
In the early 19th century, the upper river in Wyoming was part of the disputed Oregon Country. It was explored by trappers from the North West Company and Hudson's Bay Company, such as Donald Mackenzie who pioneered the area, from 1819. In 1825, the American William Ashley and party of American explorers floated down the river from north of the Uintah Mountains to the mouth of the White River. The valley of the river became increasingly used as a wintering ground for American trappers in the next decades, with trading posts established at the mouth of the White near Whiterocks, Utah, and in Browns Park.
The region was explored by John C. Fremont on several of his expeditions in the 1840s. Fremont corrected the cartographic error of Miera, establishing firmly that the river did not drain the Great Salt Lake. In 1869, the river was surveyed and mapped by John Wesley Powell as part of the first of his two expeditions to the region. During his two voyages in 1869 and 1871, he and his men gave most of the current names of the canyons, geographic features, and rapids along the river.
From the 1840s through the 1860s hundreds of thousands of emigrants made their way west along the Oregon, California and Mormon emigration trails. Nearly all of the primary emigration routes had to cross the Green River at some point. The main trail crossed near where the Big Sandy River joins the Green River in Wyoming. The river was too big and deep to ford, and was the largest and most dangerous river crossed by the Oregon Trail. for that reason, ferries were commonly run on this stretch of the river. Some of the popular ferries included the Lombard and Robinson ferries at the main crossing, and the Mormon, Mountain Man, and Names Hill ferries where the popular Sublette-Greenwood cutoff forded the river further upstream.
In 1878 the first permanent settlement in the river valley was founded at Vernal by a party of Mormons led by Jeremiah Hatch. The settlement survived a diphtheria epidemic its first winter, as well as a panic caused by the Meeker Massacre in Colorado. The town is currently the largest in the Green River Valley.
Most of the land in the valley of the river today is owned and controlled by the federal government. Private holdings are largely limited to bottoms. Until the 1940s, the economy of the valley was based largely on ranching. Tourism has emerged as the dominant industry in the region in the last several decades.
The Green River Basin contains the world's largest known deposit of trona ore near Green River, Wyoming. Soda ash mining from trona veins 900 and 1600 feet (300 and 500 m) deep is a major industrial activity in the area, employing over 2000 persons at four mines. The mining operation is less expensive for production of soda ash in the United States than the synthetic Solvay process, which predominates in the rest of the world.
The area has been mined for uranium.