THE LONGITUDINAL PROFILE OF THE COLORADO RIVER ON THE COLORADO PLATEAU: HOW DEEP IS ALLUVIUM BENEATH THE RIVER?

The Colorado River on the Colorado Plateau is commonly assumed to be directly eroding a bedrock bed. Our work suggests that the river presently flows on substantial alluvial fill in much of Marble, Grand, and Cataract Canyons; bedrock is closer to river level in former Glen Canyon. Bed elevation in reaches with the deepest fill is maintained by debris flows from tributaries, which form and maintain profile convexities at tributary junctures. Longitudinal-profile convexities occur on spatial scales ranging from rapids (order of 1-3 km) to large-scale bulges (up to 120 km). Differences between reconstructed bedrock and surveyed surface profiles suggest the fill thickness is 30-45 m in parts of Marble and Grand Canyons and 50 m in Cataract Canyon. Measurements that can be used to test these fill-thickness estimates consist of bore-hole data collected to evaluate potential dam sites and geophysical data; in addition, bedrock lined the channel (fill depth=0) at points in lower Cataract Canyon and upper Glen Canyon. Four bore holes drilled 0.5 km downstream from the confluence of the Green and Colorado Rivers in 1914 found 38 m of fill above talus. A maximum fill depth of 32 m was measured at Glen Canyon Dam, 25 river km upstream from the start of Marble Canyon. Holes drilled at four sites (river miles 33, 40, 236, and 240) in Marble and Grand Canyons between 1942 and 1952 indicate maximum fill depths of 12-27 m. Seismic data from sites in Cataract and Grand Canyons generally suggest deeper fill over bedrock; seismic data are not co-located with bore holes. At Spanish Bottom (river mile 214), three seismic refraction lines indicated a maximum depth to bedrock of 70 m just upstream from the beginning of rapids in Cataract Canyon. Other seismic-refraction data indicate maximum fill depths <45 m, with most between 0-30 m, beneath selected sand bars in Grand Canyon. In concert, these data verify that substantial alluvial fill is present beneath the Colorado River. Local base-level rises at tributary junctures near the heads of Marble and Cataract Canyons cause aggradation as much as 80 km upstream. Modeled coarse-grain inputs from tributaries suggest that, if historic frequencies of debris flows are constant over the Holocene, tributary inputs could aggrade the river channel by an average of 22 m at each of 735 junctures over a simulated 11,000 years.