Pleistocene megaflood discharge in upper Grand Coulee, Channeled Scabland, USA

Bedrock erosion and canyon formation during extreme floods have dramatically altered landscapes on Earth and Mars. A prominent example is the Channeled Scabland, a landscape in the northwestern USA characterized by deeply-incised canyons in the columnar-jointed basalt bedrock carved by outburst floods from Pleistocene glacial Lake Missoula. Quantifying paleo-discharge of the floods responsible for incising canyons is required to understand how landscapes evolve in response to extreme events, but there are few constraints on the magnitude of the floods that incised Grand Coulee, the largest canyon complex in the Channeled Scabland. The upper Grand Coulee canyon formed by headward erosion of a waterfall as basalt columns were toppled by floods spilling over the waterfall's edge. The waterfall retreated upstream until it eroded through the drainage divide separating Grand Coulee from the Columbia River. To quantify paleo-flood discharge at different stages of canyon incision, we numerically simulated floods over both the present-day topography and topography reconstructed to include the partially-retreated waterfall, and constrained paleo-discharge by matching modeled flood extents to published high-water evidence. When upper Grand Coulee was incising via headward waterfall retreat, the minimum discharge that surpasses high-water marks upstream of the reconstructed waterfall is 2.6x106 m3 s-1, which produces shear stresses great enough to drive retreat of the waterfall via toppling of basalt columns. The maximum possible discharge, which may have been triggered when a Missoula flood caused the waterfall to retreat entirely through the drainage divide impounding glacial Lake Columbia upstream, was 7.6x106 m3 s-1. The canyon-forming and maximum discharges are >80% and ~50% lower, respectively, than the 1417x106 m3 s-1 discharge predicted by assuming the present-day topography was inundated to the elevation of high-water marks. Due to bedrock incision, high-water marks may overestimate paleo-flow depth in canyons carved by floods, and bedrock erosion should be considered when estimating paleo-discharge in flood-carved canyons. Our results indicate that outburst floods with discharges and flow depths much lower than those required to inundate high-water marks are capable of carving deep canyons.