LATE QUATERNARY INCISION RATES AND DRAINAGE EVOLUTION OF THE CONFLUENCE OF THE UNCOMPAHGRE AND GUNNISON RIVERS BASED ON TERRACES DATED WITH LAVA CREEK B ASH, WESTERN COLORADO

Late Quaternary incisional and erosional history of western Colorado is documented by river terraces along the Colorado, Gunnison, and Uncompahgre rivers that contain the Lava Creek B ash (640 ka). Important new ash localities have been found directly overlying terraces just downstream from the present confluence of the Gunnison and Uncompahgre rivers at Kelso Gulch. Based on height of straths and presence of Lava Creek B ash, these terraces are correlated with: 1) terrace sequences recognized in the Uncompahgre River valley all the way to the San Juan Mountain front, 2) mantled pediments from Grand Mesa that grade to the Gunnison River, and 3) a paleotributary of the Gunnison River at Bostwick Park. Provenance analysis of terrace gravels allow the distinction between paleo Gunnison river gravels that contain granite and gneiss clasts from paleo Uncompahgre River gravels that contain Uncompahgre Quartzite and San Juan Volcanics. Distribution of different terraces near Kelso Gulch indicate that the confluence between the Gunnison and Uncompahgre rivers has shifted laterally due to several stream piracy and drainage reorganization events since 640 ka.

Strath heights of dated terraces provide excellent new incision rate data points for the upper Colorado River system. The paleo Uncompahgre- Gunnison River confluence is well exposed at Kelso Gulch. Here the Gunnison River has incised 86m in 640 ka at an average rate of 134m/Ma. Just upstream, at Sawmill Mesa, the Gunnison River has incised 106 m in 640 ka for a rate of 146 m/Ma. This dated mainstem terrace also anchors the projection of Lava Creek B- bearing pediment surfaces on Petrie and Paradox Mesas and provides the best incision rate data (134-146 m/Ma over 640 ka) so far for this reach of the Gunnison River. This age for this terrace sequence is also correlative to moraines and terraces around Ridgway; hence the moraines may be older than previously thought.