Quantifying outburst flood incision of bedrock canyons by coupling erosion, discharge, and sediment transport

Abstract:

Outburst floods are capable of eroding large volumes of bedrock over short timescales. Such floods were responsible for carving some of the largest canyons on Earth and Mars, yet it remains a challenge to reconstruct their discharge, magnitude of erosion, and total number. Here we focus on Moses Coulee, Upper Grand Coulee, and Wilson Creek, three canyons in the Channeled Scablands of eastern Washington, USA carved by Pleistocene floods from Glacial Lake Missoula. We reconstructed the pre-flood valley topography of each canyon by extrapolating the long profiles of hanging tributaries, and computed the volume of rock eroded. We then routed flow across the reconstructed topography to find the discharge where floods inundate high-water marks. Our results suggest flood discharges of 0.75 x 10⁶ m³s^-1 for Moses Coulee, 2.0 x 10⁶ m³s^-1 for Upper Grand Coulee, and 0.7 x 10⁶ m³s^-1 for Wilson Creek. These discharges are a factor of ~3 smaller than we calculated using the modern topography, suggesting that the high-water marks were likely emplaced during the early stages of canyon cutting, then abandoned as incision progressed. Bed shear stresses were used to estimate bedload flux, and total sediment flux was calculated from published bedload-to-suspended sediment flux ratios documented for outburst floods. Given the volume of rock eroded, our calculations indicate that 3-50 floods were needed to incise Moses Coulee, Upper Grand Coulee, and Wilson Creek, with the variability due mainly to the poorly-constrained suspended sediment load. Our estimates are consistent with stratigraphic evidence from downstream deposits that indicate the number of floods ranged from the single-digits based on gravel bars along the main flood route, to more than 40 floods based on stacked slackwater deposits. Our results highlight how coupling topographic change, fluid flow, and sediment transport provide insight into the size and number of megafloods based on the dynamics of bedrock canyon incision.