Canyon formation constraints on the discharge of catastrophic outburst floods of Earth and Mars

Abstract

Catastrophic outburst floods carved amphitheater‐headed canyons on Earth and Mars, and the steep headwalls of these canyons suggest that some formed by upstream headwall propagation through waterfall erosion processes. Because topography evolves in concert with water flow during canyon erosion, we suggest that bedrock canyon morphology preserves hydraulic information about canyon‐forming floods. In particular, we propose that for a canyon to form with a roughly uniform width by upstream headwall retreat, erosion must occur around the canyon head, but not along the sidewalls, such that canyon width is related to flood discharge. We develop a new theory for bedrock canyon formation by megafloods based on flow convergence of large outburst floods toward a horseshoe‐shaped waterfall. The model is developed for waterfall erosion by rock toppling, a candidate erosion mechanism in well fractured rock, like columnar basalt. We apply the model to 14 terrestrial (Channeled Scablands, Washington; Snake River Plain, Idaho; and Ásbyrgi canyon, Iceland) and nine Martian (near Ares Vallis and Echus Chasma) bedrock canyons and show that predicted flood discharges are nearly 3 orders of magnitude less than previously estimated, and predicted flood durations are longer than previously estimated, from less than a day to a few months. Results also show a positive correlation between flood discharge per unit width and canyon width, which supports our hypothesis that canyon width is set in part by flood discharge. Despite lower discharges than previously estimated, the flood volumes remain large enough for individual outburst floods to have perturbed the global hydrology of Mars.

A Mechanistic Model of Waterfall Plunge Pool Erosion into Bedrock

“…Our model is designed to predict plunge pool bedrock erosion from abrasion by impacting particles and does not account for erosion via plucking (Bollaert & Schleiss, 2003;Lamb et al, 2015;Robinson et al, 2001), toppling (e.g., Baynes et al, 2015;Lamb & Dietrich, 2009;Lapotre et al, 2016;Weissel & Seidl, 1997), or other erosional processes such as bedrock weathering (e.g., Haviv et al, 2010). Observations of smooth and wellpolished surfaces across a variety of waterfall plunge pools (e.g., Figure 1) suggest that abrasion is common and abrasion is often evoked or implied in studies of fluvial bedrock incision and waterfall plunge pool erosion (e.g., Gilbert, 1890;Lamb et al, 2007;Sklar & Dietrich, 2004).…”

Self‐formed waterfall plunge pools in homogeneous rock

“…Existing plunge pool erosion studies have focused on clear water scour of loose or weakly consolidated sediment [Holland and Pickup, 1976;Stein et al, 1993] and blocks [Bollaert and Schleiss, 2003], which are difficult to apply to massive rock ( Figure 1) where sediment impacts likely dominate erosion. Studies examining bedrock waterfall retreat have largely focused on jointed or fractured rock where retreat occurs via toppling of bedrock columns at the waterfall brink [e.g., Weissel and Seidl, 1997;Lamb and Dietrich, 2009;Baynes et al, 2015;Lapotre et al, 2016]. Lamb et al [2007] proposed a model for vertical plunge pool abrasion by sediment impacts; however, the model has yet to be tested and makes no predictions of lateral erosion.…”

GLOFs in the WOS: bibliometrics, geographies and global trends of research on glacial lake outburst floods (Web of Science, 1979–2016)

“…Margerison et al, 2005), and extreme outburst floods are also thought to shape the landscape of Mars (e.g. Lapotre et al, 2016). (2017) and Wilson et al (2018); note that the numbers of GLOFs documented from less settled regions are likely to be underestimated (see also the text).…”