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.