EXHUMED AND MODERN GRAND CANYON LANDSCAPES: CONTRASTS IN FORM AND PROCESS

The drainage of western Grand Canyon is exhuming a Laramide landscape that differs markedly in geomorphic detail from Neogene topography. These juxtaposed landscapes preserve the contrasts between valleys developed in humid and arid climates on the same, predominately carbonate, strata. The Cretaceous-Eocene climate was relatively wetter and more subtropical than the arid conditions that characterize Neogene time. Some Laramide paleocanyons have relief that approaches the modern canyons, up to 1200 m, yet their tributary morphology is very different. Laramide-age valleys, presumably formed under thicker soil and vegetation mantles, have a more typical dendritic aspect, comparable to old textbook views of “mature” Davisian landscapes. They contrast strongly with the modern canyons, which develop slot-like cross sections in the same massive Paleozoic carbonates. Younger Grand Canyon tributaries are headwardly eroding into the exhumed landscape and contain good examples of recent stream captures. Many modern drainage networks on the Hualapai Plateau change from dendritic to sinuous downstream, in association with increased incision. A common aspect of this transition in valley form is the development of short, highly sinuous channel reaches in massive carbonate bedrock immediately above major knickpoints. The sinuosity that forms directly above waterfalls is not the product of superposition from unconsolidated overburden, and it coincides with atypical convex gradients. Bends also show downstream migration in the form of marked channel asymmetry at sharp curves. The sinuous form is reminiscent of supraglacial streams that are associated with increased stream power and both supercritical and asymmetrical helical flow on the convex margins of glaciers. Bedrock channel sinuosity may begin at the obvious hydraulic discontinuity from confined to unconfined flow. The sinuosity may develop in response to hydraulic changes that operate to decrease channel gradient and flow velocity and reduce the contrast across hydraulic thresholds at waterfalls. This phenomenon may be worthy of more rigorous study due to potential implications for the development of sinuous bedrock channels in a more general context.