Yampa River series
The Yampa River wasn't always a tributary of the Green River. By using evidence from dry gravel deposits left by ancient rivers, geologists such as Wallace Hansen discovered that the Green once flowed east to the Mississippi River. Hansen wrote that oceans dominated this region more than 65 million years ago, when forces deep in the earth thrust the Uinta Mountains upward. The forces caused fracturing and faulting of the rock layers. At that time, the Green River flowed east toward the Mississippi, not south toward the Colorado River, as it does now.
When the Uinta Mountains began to subside, the Green was diverted south across softer rock layers, and the Yampa River developed as a tributary of the Green. Over time, the two rivers eroded the canyons that river rafters and kayakers enjoy today.