Narrower Paleo‐Canyons Downsize Megafloods.

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. Plain Language Summary: Some of the largest floods in Earth's history occurred 20–14 thousand years ago from the catastrophic drainage of glacial Lake Missoula. The floodwaters coalesced near the Columbia Gorge, where the water flowed through a single channel. Prior work focused on the Columbia Gorge to reconstruct the magnitude of these floods. However, these estimates did not account for erosion by the massive floods, which changed the topography. In this study, we used a machine learning computational technique to reconstruct the topography prior to megaflood erosion. Our results show ∼7.4 km3 of rock was eroded from gorge walls. Accounting for a narrower gorge and matching flood high‐water marks resulted in a peak‐flood discharge that is 30%–40% lower than prior estimates, thus downsizing the largest floods in Earth's history. Modeling sediment transport over a series of floods indicated more frequent intermediate‐sized floods transported most of the eroded rock, rather than the largest floods. This finding is similar to observations in rivers with sediment banks where tradeoffs exist between the size and frequency of floods in governing channel shape. Thus, it is possible that the same principles that govern the form of alluvial rivers also govern bedrock canyon formation by repeated megafloods. Key Points: Geomorphic observations and topographic reconstruction indicate the Missoula floods caused widening of the Columbia GorgePeak discharge estimates decline by 30%–40% when accounting for a narrower canyonMegafloods of moderate frequency and magnitude, rather than the largest flood, transported most of the eroded rock from the Columbia Gorge [ABSTRACT FROM AUTHOR]