The Green River Basin: A Natural History

The headwaters of the Green River seep from Mammoth, Minor, Sourdough and surrounding glaciers in the northern Wind River Mountains along the continental divide. Minor Glacier clings to the west face of Gannet Peak, Wyoming’s highest point at 13,804 feet. From this dramatic beginning, the water swells into one of the state’s biggest rivers and winds southward to the Utah border at Flaming Gorge reservoir. The roughly 4,000-square-mile Green River Basin in Wyoming is a desert where, for most of the winter, grass and shrubs poke through thin, windblown snow. In spring the river paints a stripe of bright vegetation through the otherwise high, dry sagebrush steppe, sheltering pastures and ranches along its edges. On the bluffs to either side of the river, the infrastructure of energy development juts from the sage.

High in the Bridger Wilderness – a primitive area designated by Secretary of Agriculture Arthur Hyde in 1931 and established as wilderness when President Lyndon Johnson signed the Wilderness Act in 1964 – glacial melt water trickles through countless lakes and scree fields and gathers into a stream at the bottom of a V-shaped valley. The Green River starts its journey flowing north past Square Top Mountain, a 3,500-foot-tall isolated block standing between two glacial canyons, and into the Green River Lakes at the edge of the wilderness boundary.

Downstream, the river sweeps west and then south, following a valley between the Gros Ventre and Wind River mountains and making the shape of a hook with its stem jutting out of the mountains into the upper Green River Basin. There, the Green flows past ranches and hayfields surrounded by the Bridger-Teton National Forest and then wends through a matrix of Bureau of Land Management and private land including large working ranches such as the Carney Ranch, Pape Ranches and the O Bar Y.

The Upper Green is a corridor for migration of both domestic and wild animals. Each spring, ranchers drive several thousand head of cattle from high desert mesas north and up the river 100 miles or more into the mountains and the Bridger-Teton National Forest. More than a dozen ranches of the Green River Cattle Association share a 168,000-acre grazing allotment on Bureau of Land Management land on the mesas, and a 127,000-acre allotment on the national forest. It’s a three-week trip along a traditional stock route called the Green River Drift, 24 miles of which is a fenced driveway. The cattle drift back south in the fall at the first snow. The ranchers spend another month sorting their cattle from their neighbors’, before driving them back to the home ranches.

Many species of wildlife also follow the river corridor. Between 1994 and 2003, Wyoming Game and Fish Department officials released more than 70 captive-bred trumpeter swans in the upper Green River. The birds follow the current downstream to find milder weather for the winter and then return to the upper stretches along the Gros Ventre and Wind River Mountains for spring breeding and nesting. The cracked, vibrato calls of sandhill cranes ring along the upper Green in the springtime. Hardy forage and the sparse snowpack make the basin an important winter range for as many as 100,000 ungulates, including about 59,000 antelope, the largest herd anywhere. Elk, deer, and pronghorn follow the path of the river during their seasonal migrations. The Teton pronghorn, for example, follow the Green upstream each spring nearly to its headwaters before crossing the Gros Ventre Mountains to Jackson Hole, where they give birth to their fawns along the Snake River in Grand Teton National Park. Traversing 170 miles between winter and summer range, this is one of the longest recorded land animal migrations in the western hemisphere.