Contrasting bedrock incision rates from snowmelt and flash floods in the Henry Mountains, Utah

Joel P L Johnson, Kelin Whipple, Leonard S. Sklar

Research output: Contribution to journalArticle

30 Scopus citations

Abstract

Hydrograph variability and channel morphology influence rates of fluvial bedrock incision, but little data exist on these controls in natural channels. Through field monitoring we demonstrate that (1) short-term bedrock channel incision can be rapid, (2) sustained floods with smaller peak discharges can be more erosive than flash floods with higher peak discharges, due to changes in bed allu-viation, and (3) bedrock channel morphology varies with local bed slope and controls the spatial distribution of erosion. We present a three-year record of flow depths and bedrock erosion for a human-perturbed channel reach that drains the Henry Mountains of Utah, USA. Starting from a small and steep (~30% slope), engineered knickpoint in Navajo sandstone, erosion has cut a narrow, deep, and tortuous inner channel in ~35-40 years. Along the inner channel, we measured up to 1/2 m of vertical incision into Navajo sandstone over ~23 days, caused by the 2005 season of exceptional snowmelt flow. In contrast, flash floods caused little bedrock incision even when peak discharges were much higher than the peak snowmelt flow. Flash floods were net depositors of coarse sediment while snowmelt flow cleared alluvial cover. We document the formation of a pothole and interpret that it was abraded by bedload rather than fine suspended sediment. Finally, several slot canyons (Peek-a-boo, Spooky, and Coyote Gulch narrows) in the nearby Escalante River drainage basin have erosional morphologies similar to the monitored channel reach. Feedbacks between flow, sediment transport, and transient erosion provide a plausible explanation for the evolution of channel slope, width, and bed roughness of these natural bedrock channels.

Original languageEnglish (US)
Pages (from-to)1600-1615
Number of pages16
JournalBulletin of the Geological Society of America
Volume122
Issue number9-10
DOIs
StatePublished - Sep 1 2010

ASJC Scopus subject areas

  • Geology

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