American Geophysical Union Become an AGU Member
Subscribe to AGU Journals		 AGU Home AGU Publications

Read Full Article (file size: 1805798 bytes)    Cited by

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 111, F02020, doi:10.1029/2004JF000257, 2006

Effects of tributary debris on the longitudinal profile of the Colorado River in Grand Canyon

Thomas C. Hanks

U.S. Geological Survey, Menlo Park, California, USA


Robert H. Webb

U.S. Geological Survey, Tucson, Arizona, USA


Abstract

The Colorado River in Grand Canyon has long been known as a “rapids-and-pools” river, with the rapids owing their existence primarily to tributary debris flows. The debris flows deposit subaerial debris fans that constrict the channel laterally and, when they enter the river, raise the bed elevation. The rapids are short-wavelength (∼0.1 to ∼1 km), small-amplitude (≤ ∼5 m) convexities in the river's longitudinal profile, arising from the shallow gradient in the upstream pool and the steep gradient through the rapid itself. Analysis of the entire longitudinal profile through Grand Canyon reveals two long-wavelength (∼100 km), large-amplitude (15–30 m) river profile convexities: the eastern canyon convexity between river mile (RM) 30 and RM 80 and the western canyon convexity between RM 150 and RM 250. Convexities of intermediate scale are also identified in the longitudinal profile. These longer-wavelength, larger-amplitude convexities have strong spatial correlations with high rates of debris flow occurrence, high densities of Holocene debris fans, the largest debris fans along the river, and alluvial thicknesses of 10 m or more. River profile convexities are unstable and require an active and powerful geologic process to maintain them, in this case the abundant, frequent, and voluminous Holocene debris flow activity in Grand Canyon. At all wavelengths the most likely cause for these river profile convexities is Holocene aggradation of the riverbed beneath them, driven by the coarse particles of tributary debris flows. Large enough debris flows will slow river flow for kilometers upstream, causing it to drop much of its suspended load. Integrated over time and all of the tributary point source contributions, this process will build short-wavelength convexities into long-wavelength convexities. For most if not all of the Holocene the Colorado River has been dissipating most of its energy in the rapids and expending the remainder in transporting fine sediment through Grand Canyon, with little or no regional incision of bedrock.

Received 22 October 2004; accepted 30 January 2006; published 17 June 2006.

Keywords: Colorado River Basin; Grand Canyon; river profile convexities.

Index Terms: 1810 Hydrology: Debris flow and landslides; 1825 Hydrology: Geomorphology: fluvial (1625); 1856 Hydrology: River channels (0483, 0744); 1861 Hydrology: Sedimentation (4863).

Read Full Article (file size: 1805798 bytes)    Cited by

Citation: Hanks, T. C., and R. H. Webb (2006), Effects of tributary debris on the longitudinal profile of the Colorado River in Grand Canyon, J. Geophys. Res., 111, F02020, doi:10.1029/2004JF000257.