STREAM RESPONSE TO BASE-LEVEL CHANGE ALONG THE ROCKY MOUNTAIN FRONT IN COLORADO AND WYOMING

The boundary separating the front ranges of the Colorado-Wyoming Rockies from the Colorado Piedmont is an escarpment that formed during the Neogene due to differential erosion between the sedimentary rocks of the plains and the crystalline rocks of the mountains. The excavation of the sedimentary cover from the plains has produced a local base-level fall for streams draining the crystalline rocks, whose magnitude increases from nearly zero in the “Gangplank” area of southern Wyoming to ~1000 m in the Wet Mountains of south central Colorado. In order to study the response of streams to this spatially variable change in base-level fall, stream profiles were extracted from 30m DEMs, knickpoints on the profiles were identified, and concavity and steepness indices were calculated for 20 streams draining eastward from the Laramie and Rampart Ranges and the Wet Mountains onto the piedmont. Pronounced knickpoints were identified on all streams 1 to 8 km upstream from the mountain front. Steepness indices, normalized to a concavity of 0.45, range from 44.5 to 266. Results show a positive correlation between base-level change and steepness index, but no clear correlation between knickpoint migration distance and base-level change. These results suggest that 1) the perturbation leading to base level change and knickpoint initiation may have been spatially contemporaneous; and 2) steepness index may be a proxy for relative base-level change in this setting. Alternatively, if steepness indices are used purely as a proxy for rock uplift rates, these results show a trend of greater rock uplift to the south, which may have been the driving force for variable degrees of base-level change.