EPISODIC INCISION OF COLORADO RIVER IN GLEN CANYON, UTAH

We calculate episodic incision rates of the Colorado River based on absolute ages of two levels of Quaternary deposits adjacent to Glen Canyon, Utah along the north flank of Navajo Mountain. Minimum surface ages are determined by a combination of cosmogenic radionuclide surface exposure ages, uranium-series and soil-development formation times. Bedrock incision rates of the Colorado River between ~500 ka and ~250ka, and ~250 ka to present are ~0.4 m/ka and ~0.7 m/ka, respectively. These rates are more than double the rates reported in the Grand Canyon, suggesting that the Colorado River above Lees Ferry is out of equilibrium with the lower section of the river. We also determine incision rates of two tributaries to the Colorado River. Oak Creek and Bridge Creek flow off Navajo Mountain into Glen Canyon from the southeast. Oak Creek and Bridge Creek both have incision rates of ~0.6 m/ka over the past ~100 ka at points about 9 km away from the main stem of the Colorado River.