The Southern Rocky Mountains are generally divided from the Western Rocky Mountains by the Green River and the Colorado River below the Green River. The Southern Rockies are divided from the Central Rocky Mountains by South Pass in Wyoming and the drainage running east from the pass down the Sweetwater River and the North Platte River; and the drainage running southwest from the pass down Pacific Creek and Sandy Creek to the Green River. This divide between the Southern Rockies and the Central Rockies provided the lowest elevation traverse of the Rocky Mountain region for the historic Oregon Trail, the Mormon Trail, and the California Trail. The southern end of the Rocky Mountains are considered to be the Jemez Mountains and the Southern Sangre de Cristo mountains of New Mexico. Mountains south of here in N.M. are classified as the Arizona/New Mexico Mountains using the EPA Level III Ecoregions System.