The Yampa River, 250 miles in length, rises along the west flank of the Park Range, flowing north to Steamboat Springs, Colorado, and then turning westward, eventually merging with the Green River in Dinosaur National Monument. When the river formed, Tertiary deposits covered most of northwestern Colorado, a mix of erosional debris, windblown sand and volcanic ash. Intrusive volcanic plutons and buried anticlines of Precambrian and Paleozoic rocks were spaced throughout this Tertiary blanket, later uncovered by the Yampa and its tributaries.
Today, the Upper Yampa has stripped the Tertiary deposits from the underlying Cretaceous strata; that portion of the river runs atop Mancos shale, producing rich farmlands as far west as Hayden. From there to Craig, the Yampa enters a plateau of Cretaceous Mesaverde sandstone and its valley of shale gradually narrows. West of Craig and through Maybell, the river carves its valley through the Browns Park Formation, composed of eolian Tertiary sandstone, siltstone and Tertiary volcanic debris. Further west, the Yampa appears to have drilled its way through the Cross Mountain ridge, an uplift of Precambrian rock flanked by Mississippian limestone; in reality, the river first became entrenched in overlying Tertiary sediments, cutting down through the ridge and removing the younger sediments in concert.
Beyond Cross Mountain, the Yampa enters the eastern portion of Dinosaur National Monument, carving a long, winding canyon through Weber Sandstone (Permian in age) before joining the Green River in Echo Park. Though its broad, rich valley near Steamboat Springs is well known to hordes of skiers and summer visitors, the Yampa River watershed harbors some of the most remote and least traveled landscapes in Colorado, a region of stark beauty and home to the State's largest herds of North American pronghorn.