Geomorphic Response to Extreme Flooding in Northern Yellowstone National Park, USA

Understanding and predicting flood-induced geomorphic change, and the relative influences of fluvial forces and valley-bottom geometry on system response, are persistent quandaries in geomorphic-process studies. We combined field surveys, remote sensing, and hydraulic modeling to assess the geomorphic effects of the June 2022 flooding in northern Yellowstone National Park, in the western United States, which was by far the largest recorded flood at nearby gaging stations and caused extensive damage to park infrastructure. We evaluate geomorphic response relative to two ways of calculating the hydraulic forcing produced by the flood: unit stream power and impulse, a metric that incorporates flow duration, grain size, channel-bed slope, flood depth, and a threshold based on threshold channel theory. Measurements of pre- and post-flood active-channel width change captured the geomorphic response associated with deposition- and erosion-related channel migration. The geomorphic response to flooding showed substantial variability. Notable lateral channel erosion in some cases occurred in areas of high flow strength, such as high-gradient reaches, but elsewhere was concentrated along erodible channel margins and/or downstream of confluences, where calculated flow forces were unremarkable. Channel widening was greater in partially confined reaches than in unconfined or confined reaches, and channel areas of extensive deposition were associated with decreases in valley-bottom confinement. Low-gradient, unconfined reaches where flood flow spread across floodplains showed little evidence of the passage of a historic flood. As climate change increases the propensity for hydroclimatic extremes and events such as the Yellowstone flood, investigation of links between flood forcings and geomorphic response is particularly important.