Reports from the fur trappers who first entered the mountains and those following mention the trail through the pass as being in long term us by the Shoshone, Bannack, Arapaho, Gros Ventre, Flat Head or Bitterroot Salish, Nez Perce, Crow and others. The pass provide access into three river sheds. An east-west route ran from the east up the Wind River valley and provided a western route by the Gros Ventre or Hoback Rivers to the Valley of the Snake River in Jackson's Hole. To go south, the traveler would follow the Green River. The Union Pass Route was an extension of a North South Route up the Valley of the Yellowstone. Coming up the Valley of the Thoroughfare, west of the Absaroka Mountains and south of Yellowstone Lake, the trail followed the North Fork of the Yellowstone. Crossing over Two Ocean Pass, the trail split east and west around Terrace Mountain into the Wind River Valley at DuNoir. From Union Pass south bound travelers could use the Green River Valley to connect east through South Pass or west toward the Snake River.