COMPARISON OF THE MODERN PROFILES AND DISCHARGE OF THE GREEN AND COLORADO RIVERS AND IMPLICATIONS FOR EPEIROGENIC UPLIFT OF THE COLORADO PLATEAU AND ROCKY MOUNTAINS

We compare the two major rivers draining the western slope of the Rocky Mountains, the Green River and the upper Colorado River. This comparison is motivated by the hypothesis that epeirogenic uplift of the Colorado Rockies above the Aspen anomaly may have influenced the upper Colorado River drainage more than the Green River. Slope-area calculations indicate that the upper Colorado is systematically steeper for a given drainage area than the Green above their confluence. The basic geology varies widely between the rivers and along their respective courses, but both traverse a mixture of soft lithologies and harder rocks.The longitudinal profiles of the rivers reveal starkly different knick-zone geometry. The Green River has two distinct knickzones, and the Colorado system has more subtle changes in gradient overall, but also has knickzones in Gore Canyon and on the Gunnison River in Black Canyon. Both of these canyons are incised into basement. Differential incision rate data suggest the knickzones are transient and may be related to ongoing drainage adjustments. From slope-area data alone, it might be interpreted that the Green River has a gentler slope due to a greater discharge. However, comparison of historic discharge records plotted relative to drainage area shows that the Colorado River has greater discharge per given drainage area. We interpret the combined data showing both steeper slopes and higher discharge for the Colorado as favoring models for post-Miocene uplift of the Colorado Rockies.

New surficial mapping in Desolation/Gray Canyons, Utah reveal multiple terrace levels, with more terraces preserved upstream of the knick-zone than downstream. Surveying yielded strath heights as much as 145 m above the modern river including large abandoned meanders. The gravel thickness throughout these terraces ranged from strewn pebbles on bedrock benches to 30 m deep piles of gravel, with most around 5 m thick. New gravel samples from the terraces in Desolation/Gray Canyons and elsewhere on the Colorado and Green are being processed for burial cosmogenic dates. Incision rates from cosmogenic work coupled with previous incision rate data and analyses of stream geometry and discharge should provide a comprehensive dataset for evaluating the response of these rivers to lithologic effects, climate change, stream integration and epeirogenic uplift.