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.

Shortly after entering Utah, the Green enters Flaming Gorge, the first of a long series of canyons. Flaming Gorge, Horseshoe, and Kingfisher canyons are short but scenic. Red Canyon, the next in the series, is about thirty miles long, and is now the site of Flaming Gorge Dam. After Red Canyon, the Green enters Browns Park, a large east-west trending basin, and flows through it for fifteen miles before crossing the Colorado border.

The river flows through the northwest corner of Colorado for forty miles, through the Canyon of Lodore, and receives the waters of its largest tributary, the Yampa, while in Colorado. The Green re-enters Utah in the middle of Whirlpool Canyon, about five miles below its confluence with the Yampa. After passing through Island and Rainbow parks, the Green runs a short but turbulent seven miles through Split Mountain Canyon, which has the greatest fall of any of the canyons of the Green--almost twenty-one feet per mile--and consequently has some of the most difficult rapids on the entire river. Below the mouth of Split Mountain Canyon, the Green flows through the open, arid landscape of the Uinta Basin for more than 100 miles, unconfined by canyons and undisturbed by rapids. In the Uinta Basin two more large tributaries, the Duchesne River from the west and the White River from the east, join the Green, their mouths almost across from each other.

The next canyons are Desolation Canyon and Gray Canyon, two back-to-back canyons that total 120 miles in length. Desolation is the deepest and longest of the canyons of the Green, while Gray (earlier known as Coal Canyon) is lower but narrower. The Green leaves Gray Canyon just above the town of Green River, Utah, and flows through an open area for about thirty miles before entering the last of the canyons it traverses, Labyrinth and Stillwater canyons. As the names suggest, these are quietwater canyons, where the river loops in sinuous curves around towering cliffs of sandstone. The Green meets its sister stream, the Colorado, at the end of Stillwater Canyon, in the middle of what is now Canyonlands National Park.

The Green traverses several different vegetation and fauna zones during its course through Utah, ranging from high mountains in the north to slickrock deserts in the south. Pines, firs, and groves of aspen are common in the higher parts, while pinyon and juniper are predominant below the mountains. In the lower elevations, shadscale, sagebrush, cactus, and desert shrubs are most common. Cottonwoods, tamarisk, and willows are the predominant members of the riparian plant community throughout the river's length. Likewise, fauna follow typical life zones. Elk, deer, bighorn sheep, coyotes, rabbits, squirrels, and other small rodents, and an occasional bobcat or cougar are found along the Green. Snakes, lizards, toads, and other reptiles are common near the river, less common away from it. Bird life, especially along the river corridor, is abundant, as the river is part of the main north-south flyway for some waterfowl.

The flow of the Green varies from season to season and from year to year, based on the amount of snow that accumulates in the upper parts of the drainage basin. Since the basin is largely arid, only a small portion of the total precipitation reaches the mouth of the river. The Green has only one large dam in its entire length, and so is still largely a wild river. In other words, the flow of the river can be drastically affected by sudden changes in temperature, or by rainstorms over the drainage of its tributaries. In the spring, when the snowpack is melting, the Green can flood, while during the later summer months it has been known to all but dry up.

Evidence of ancient inhabitants abounds in the Green River Basin. The basin was home to the Fremont Culture, which flourished in the tributary canyons and in sheltered areas from about A.D. 600 to around A.D. 1200. The Fremont were a semi-nomadic people, who made distinctive pottery and figurines, used atlatals, and lived in pithouses. They are best known for their rock art, found on canyon walls and in sheltered overhangs throughout the river basin. The lower stretches of the Green formed the northern boundary of the Anasazi culture area, and therefore evidence of their occupation of the Green River area is limited. In later years, Shoshone and Ute peoples, both nomadic hunters, occupied the basin of the Green, the Shoshone to the north of the Uinta Mountains and the Utes to the south. The Utes still live near the river; their reservation is in the Uinta Basin.

Just as the Green is Utah's major stream, so it was featured in the earliest written account of Utah's landscape. In September 1776 Friars Dominguez and Escalante and their companions crossed the Green on their way to the Utah Valley. Escalante left a written account, and the expedition's map-maker, Don Diego Miera y Pacheco, drew a map which showed an erroneous course for the river.

Other Spaniards and, later, Mexicans, also were familiar with the Green; the old Spanish Trail from New Mexico to California crossed the Green just above the present-day town of Green River, Utah. These later explorers, who were probably traders, prospectors, and slavers, left no written records mentioning the river.

Although trapping parties from the Hudson's Bay Company were in the upper basin of the Green as early as 1819, it wasn't until 1825 that American trappers explored the river in Utah. In April of that year, William Ashley and a party of trappers floated down the river from north of the Uinta Mountains to the mouth of the White River. This marked the first recorded time that anyone had actually floated on the river. In the next decade, Browns Park and the bottoms around the mouth of the White became favorite wintering grounds and places of rendezvous for the trappers, as they also had been for the Indians before them. Several trading posts were established in the basin of the Green, at the mouth of the White, near Whiterocks, Utah, and in Browns Park by Antoine Robidoux, Denis Julien, and others. Julien was also one of the few trappers to actually float the river after Ashley's pioneering voyage. Most refused that means of transportation as John C. Frémont, who explored the area around the upper Green in 1843, noted: "Though offering many temptations, and often discussed, no trappers have been found bold enough to undertake a voyage with so certain a prospect of a fatal termination."

The California gold rush of 1849 was the motivation for the next party to explore the Green River. William Manly and several companions entered the Green near the Sweetwater crossing, and floated in an abandoned ferryboat and later dugout canoes all the way to the Uinta Basin. Above the mouth of the White River, Manly met Wakara, chief of the Utes, who convinced the '49ers that the Green was not the easiest route to California, as they had thought. Manly and his party left the river and journeyed overland to Salt Lake City. The Mormons, who settled Salt Lake in 1847, sent exploring parties into the Uinta Basin as early as the 1850s, but the surveyors returned with unfavorable reports, and the basin of the Green remained unsettled by the Latter-day Saints for another twenty years. In the meantime, the Green River basin was acquired by the United States from Mexico through the Treaty of Guadelupe-Hidalgo in 1848.

It was not until 1869 that the Green was surveyed and mapped by a scientific party. John Wesley Powell embarked on the first of two voyages down the Green in May 1869 and floated the river all the way to its confluence with the Colorado and beyond. Powell left a detailed account of the river and the surrounding landscape and prepared the first thorough maps of the river basin. Powell left his mark on the in other ways as well. He and his men named most of the canyons, geographic features, and rapids along the Green River during his two voyages in 1869 and 1871. Powell also paved the way for later generations of explorers and scientists interested in the unique geology of the basin of the Green River.

Tourism also has become a major factor in the economies of many towns in the basin. Places such as Dinosaur National Monument and the Cleveland Lloyd Dinosaur Quarry (administered by the Bureau of Land Management), as well as Flaming Gorge and other reservoirs, and of course the river itself, draw thousands of tourists from all over the world. The river basin is crossed by several major transcontinental highways and railroads, chief among them being U.S. highways 40 and 6/50, and the Denver and Rio Grande Western Railway. Although there have been attempts, most notably around the turn of the century, to develop passenger and freight service on the Green River, the seasonal flows and rapid-filled stretches of the river have precluded any such development.

As early as 1904 the U.S. Reclamation Service and the state government began investigating the possibility of building dams on the Green for water reclamation and power production. A comprehensive survey of the Green River for dam sites was undertaken by the U.S. Geological Survey and Utah Power and Light Company in the years 1914 to 1922. Shortly after World War II, the U.S. Bureau of Reclamation announced plans to build a large dam on the Green River in Dinosaur National Monument, just inside the Colorado border. Another was planned for Split Mountain Canyon, a few miles downstream. Widely praised in Utah, this plan soon met fierce opposition from conservationists, resulting in a bitter, protracted, and ultimately successful fight to defeat the Echo Park Dam. The controversy left divisions in the communities of the Green River Basin that linger to this day. In 1956 work began on a dam in Red Canyon, on the Upper Green. Flaming Gorge Dam was completed in 1963, and today the reservoir has become a popular destination for fishermen and boaters.

The Green River is the largest of all of Utah's streams and is central to the history of the state in terms of its exploration and development. Therefore, the Green fully deserves to be called Utah's master stream.