ORIGIN OF THE UPPER COLORADO RIVER SYSTEM: THE VIEW FROM WESTERN COLORADO

When and how the Colorado River system originated are key questions regarding the evolution of Rocky Mountain and Colorado Plateau landscapes. While the upper Colorado River system has received less attention than the lower Colorado system, upper Colorado River deposits provide critical insights on the origin of the Colorado River.

Previous studies indicate that major west-flowing rivers draining the Rocky Mountains in western Colorado did not exist until around 10 Ma. Evidence for the earliest of such rivers is represented by high-elevation river gravels that contain granitic clasts probably derived from Precambrian portions of the Sawatch and/or Gore Ranges. The best known example of a ~10 Ma ancestral Colorado River is at Lookout Mountain near Glenwood Springs. Rounded river gravels are present at an elevation of 2.6 km, and are dated to ~10 Ma. The gravels include clasts of Precambrian granite that were transported westward at least 80 to 100 km. Precambrian granitic river gravels also occur at Flat Top Mountain near Gunnison at an elevation of ~3.2 km beneath ~10 Ma basaltic lava flows. The granitic gravels were most likely derived from the Sawatch Range located east of Flat Top Mountain.

Late Miocene (~10 Ma) basaltic volcanism was relatively widespread in western Colorado; the flows provide clues to the ages of these ancient river gravels. The basalt is typically found at present-day elevations of 2.9 to 3.2 km. The fact that these ~10 Ma flows are found at similar elevations in the region suggests that low-lying portions of the late Miocene landscape exhibited less relief than that of today. The basalt flows also provide a temporal datum (~10 Ma) from which long-term incision rates of major rivers such as the Colorado and Gunnison can be estimated. Calculated river incision rates range from ~70 to 150 m/Ma, and in general, incision rates decrease upvalley towards major mountain ranges. This pattern suggests that drainage development has been accomplished by headward incision. This process could be explained by either 1) long-term drainage integration as the Colorado River responded to opening of the Gulf of California, or perhaps 2) broad uplift of the Rocky Mountain region, and initiation of transient knickpoints along the flanks of the uplift zone.