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

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.