DISCHARGE RECONSTRUCTION OF CANYON-CARVING OUTBURST FLOODS FROM THE CORDILLERAN ICE SHEET (Invited Presentation)

The Channeled Scablands of eastern Washington contain deep bedrock canyons carved into fractured basalt by catastrophic Pleistocene floods triggered by the failure of ice-dams that impounded Glacial Lake Missoula. Larger, morphologically similar canyons have been used to make inferences regarding flood discharges and paleoclimate on Mars. Reconstructing the magnitude of the canyon-forming floods is challenging, but essential for understanding the paleo-environmental conditions during times that floods significantly altered planetary surfaces, and for understanding whether such floods may trigger abrupt climate change. A common method for estimating flood discharges assumes that floods filled the canyons brimful to high water markers, which is appropriate only for canyons that existed prior to flooding and underwent no erosion during extreme flows. However, canyon topography co-evolves with flood hydraulics; hence the brimful flow assumption yields maximum estimates of paleo-discharge. An alternate hypothesis proposes that canyon morphology adjusts during incision such that bed shear stresses do not greatly exceed the threshold for erosion. Here we focus on Moses Coulee, which was carved by the Missoula Floods. By combining hydraulic flood simulations and thresholds for bedrock plucking, we show that accounting for erosion thresholds during canyon incision results in near constant discharges and flood depths that are 15 to 40% of brimful estimates. The predicted discharges are consistent with flow-depth indicators from gravel bars within the canyon. In contrast, the brimful flood assumption predicts a significant and monotonic increase in flood discharge as the canyon progressively incised, which is at odds with discharges expected from the glacial lake-outburst flood source. These findings suggest that megaflood-carved landscapes in well fractured rock may evolve to a threshold state for bedrock erosion, which implies megafloods that incised canyons have significantly lower discharges than previously thought.