6 Quaternary Geology/Geomorphology I: Streams and Slopes

Sunday, 2 November 2003
8:00 AM - 12:00 PM

LONGITUDINAL-PROFILE CONVEXITIES RELATED TO LONG-TERM DEBRIS-FLOW INPUTS TO THE COLORADO RIVER IN GRAND CANYON, ARIZONA

Paper number: 6-9
Program: Technical Sessions
Time: 10:00 AM – 10:15 AM

Authors

Abstract

Rapids on the Colorado River in Grand Canyon represent the accumulation of coarse particles due to episodic debris flows. The net effect of coarse-particle inputs is apparent in the longitudinal profile of the river, originally surveyed in 1923 and measured again in 2000 using LIDAR. Individual rapids represent small-scale convexities in the longitudinal profile with spatial scales of one river kilometer or less; these features exhibit considerable changes that occur on time scales of years and decades and result from frequent debris-flow deposition and river reworking. Rapids serve as local base levels that focus the river’s energy into transport or removal of coarse sediment. Intermediate-scale convexities in the longitudinal profile are generated at large debris fans (e.g., Prospect Canyon, Nankoweap), where significant alluvial input and debris-fan reworking create multiple secondary rapids and alternating debris bars. These secondary features extend several river kilometers downstream from source tributaries. Finally, large-scale convexities, with spatial wavelengths ranging from 30-120 river kilometers are also apparent. In comparison with a straight-line gradient originally proposed by Leopold, the overall longitudinal profile shows two major and three lesser large-scale convexities. The amplitudes of these convexities have not changed historically. Two major river-profile convexities are spatially associated with high probability of debris-flow occurrence and large Holocene debris fans. The three lesser large-scale convexities are spatially associated with high debris-flow probability alone. Comparison to a straight-line gradient indicates maximum fill depths of about 30 m; previous seismic work suggests that local fill depths may be as great as 45 m. These river-profile convexities are the present-day manifestation of a Colorado River subject to episodic but persistent aggradation and subsequent incision through the mid-to-late Quaternary. In contrast to the long-held assumption that the river is currently incising into bedrock, our work suggests that the Colorado River through much of Grand Canyon is expending its energy removing small- and large-scale convexities created by alluvial fill.