Narrower Paleo‐Canyons Downsize Megafloods

Abstract

Catastrophic drainage of glacial Lake Missoula through the Columbia River Gorge, USA, produced some of the largest floods ever known. However, erosion of the gorge during flooding has not been quantified, hindering discharge reconstructions and our understanding of landscape change by megafloods. Using a neural network and geomorphic observations, we reconstructed the gorge topography and found ∼7.4 km³ of rock was eroded from gorge walls. Accounting for a narrower canyon and matching flood high-water marks resulted in peak-flood discharge reconstructions of 6 × 10⁶–7 × 10⁶ m³ s⁻¹, which are 30%–40% lower than prior estimates based on the present-day topography. Sediment transport modeling indicated that more frequent intermediate-sized floods transported most of the eroded rock. Thus, similar to alluvial rivers, discharge magnitude-frequency tradeoffs may also govern canyon formation by repeated megafloods.

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