Tributary response to baselevel fall in Grand Canyon: incision timing and influences of coarse sediment supply on knickpoint form and channel steepness

Abstract:

Western Grand Canyon is characterized by a steep inner gorge inset into broad upland plateaus. The uplands preserve sediments and volcanic rock from a period of relatively stable baselevel during the Tertiary coeval with the erosional beveling of the Hualapai Plateau to a low relief surface that cross cuts dipping erosionally resistant carbonates and weaker shale and sandstone units. Low erosion rates on the plateau surface persist, but more recent baselevel fall likely drove dissection of the plateau by the Colorado River and its tributaries. Baselevel of the western Grand Canyon region was likely lowered a net ~1000 m during basin and range extension coincident with slip on the Grand Wash fault between 18 and 12 million years ago and associated sedimentation. The tributaries draining into the Grand Wash Trough produce canyons and rugged topography, but these channels and surrounding hillslopes are not as steep as in the western Grand Canyon. Channel steepness and hillslope gradient are positively correlated with erosion rate where lithology and climate are similar and we infer that erosion rate is higher and began more recently in western Grand Canyon than along the Grand Wash Cliffs.

The tributaries of western Grand Canyon express a variety of knickpoint forms, opening several questions about the controls on channel form due to baselevel fall. Knickpoints here are often expressed at lithologic contacts however there is no predictive relationship between lithology and the magnitude or shape of an associated knickpoint. Large tributaries tend toward gentle knickpoints, with few small or no waterfalls, while small tributaries tend toward localized abrupt knickpoints with ephemeral waterfalls reaching over 100 m in height. We hypothesize the large, waterfall-style knickpoints may be fluvial hanging valleys and are exploring whether they are associated with a threshold contributing drainage area. However, variable contributions of sediment type and size may significantly influence knickpoint evolution by setting drainage area thresholds. New analyses presented will explore the complex relationships between knickpoint form and sediment metrics.