LATE PLEISTOCENE COLORADO RIVER TERRACES, WESTERN COLORADO: A TEST OF THE STREAM POWER MODEL

Geologic mapping reveals a flight of 6 down-stepping strath terraces of the Colorado River near Grand Junction, Colorado. Optically stimulated luminescence (OSL) dating has been completed for 2 of the 3 youngest Colorado River terraces. The oldest dated terrace gravels are capped by a Stage III calcic soil, have a strath that is ~23 m above the modern floodplain, and range in age from 64 to 68 ka. The youngest dated terrace gravels are buried by 3 to 12 m of tributary alluvium, have straths that are slightly beneath (-2 m) the modern floodplain, and range in age from 14 to 24 ka. A sample from the base of the gravels beneath the nearby Gunnison River floodplain yielded an age of 11.4 ka at a depth of 8 m below the floodplain surface.

Stratigrahic relationships and the OSL ages suggest that abandonment of the oldest dated Colorado River terrace ca. 65 ka was accompanied by 24 to 30 m of bedrock incision. Subsequent aggradation produced an undated fill terrace that was subsequently incised resulting in an additional 2-3 m of bedrock incision by ca. 24 ka. Gravel accumulation and strath formation ended ca. 14 ka with shifting of the Colorado River southward to its present-day position and burial of the youngest terrace surface by tributary alluvium. The OSL age from beneath the Gunnison River floodplain suggests that this shift in river course was accompanied by a final phase of modest (2-3 m deep) bedrock incision between 14 and 11 ka. Collectively, these data indicate that 28-36 m of Colorado River bedrock incision occurred between 65 and 11 ka, and that gravel deposition, strath formation, and bedrock incision occurred primarily during glacial hydrological conditions, similar to previous results for the Laramie River in Wyoming. Downstream in the eastern Grand Canyon, the 64 to 71 ka Colorado River terrace that chronologically overlaps with the oldest dated (64-68 ka) terrace in this study, has a strath that is locally buried beneath the modern floodplain. This observation indicates that there has been a significantly greater amount of Colorado River bedrock incision upstream (28-36 m) than downstream (<10 m) over the past ca. 65 ka, although the long-term incision rates are similar (~140 m/my).