ROCK-STRENGTH CONTROL IN DESOLATION & GRAY CANYONS OF THE GREEN RIVER: A RIVER LANDSCAPE IN DYNAMIC EQUILIBRIUM

Geologists intuitively know that the physical and chemical weathering characteristics of bedrock are basic controls on landscape form and process. This is particularly evident in the weathering-limited and physical-weathering-dominated canyon country of the Colorado Plateau, yet little is known about the relation between bedrock strength and canyon geomorphology. Our research is the first to quantify the relation between bedrock type and strength, hillslope processes, and the morphometry of a major bedrock canyon.

Desolation and Gray Canyons of the Green River in Utah were split into four study reaches. The upper reach is characterized by abundant shale and thin sandstone, the second by massive, cliff-forming sandstone, the third by mixed mudstone and limestone, and the lowest reach by cliff-forming sandstone and subordinate interbedded shale. Rock mass strength (RMS) of all major units was calculated using Schmidt-hammer measurements of intact rock strength combined with pertinent field observations. Detailed mapping of four similar-sized tributary drainages representative of each reach has been done to characterize geomorphic processes and history as well as estimate shale percentage for rock strength estimates. GIS-based measurements of drainage area, debris-fan area, drainage basin slope, fan area/drainage area, and slope/drainage area were analyzed in the context of RMS and mapping results.

Results confirm intuition by indicating RMS is negatively correlated to canyon-bottom width and normalized debris-fan size, whereas shale percentage is positively correlated to drainage slope. Erodability is dominated by jointing characteristics rather than inherent elastic strength, as is common in RMS studies. In Desolation and Gray Canyons there is a balance between rock erodability, slope, and river geomorphology, consistent with a landscape in dynamic equilibrium. This suggests that possible climate and neotectonic influences in the study area are minor. These fundamental data should help scientists better understand geologic controls on river dynamics and thus aid in wildlife habitat, river restoration, and recreation management.