Mapping Extreme Discharge in the Yellowstone Watershed During Historic Flooding in 2022

Abstract

On June 13, 2022, Yellowstone National Park (YNP) experienced a historic flood event. The proximate cause for flooding was heavy rain on a late snowpack. An atmospheric river storm hit YNP discharging a record amount of rain on melting snow in a short period, causing extreme flooding in the region. In YNP, the watershed received a combined 7.5-9.5 inches of rain and snowmelt in a 24-hour period. Because this type of event is relatively rare in the Greater Yellowstone region, and the June 2022 event is recent, many questions remain including ones as basic as how extreme and extensive it was. Initial evaluations suggested that the recent flooding was unprecedented, but judgments were informal or preliminary. Therefore, I quantified the relative magnitude of the Greater Yellowstone region flooding in 2022 using hydrological records. The goals of my project were to analyze hydrologic data to characterize the magnitude of event runoff in YNP and surrounding areas and link the hydrological response to the meteorology of the type of severe rain-on-snow event that caused it. To evaluate the severity and spatial extent of extreme runoff during the June 2022 flood event, I analyzed USGS discharge data for stream gauges in YNP and the Greater Yellowstone region. I evaluated 139 gauges with long records to identify ones with extreme peak flows in June 2022 compared to historic peaks, selected a representative subset with extreme or non-extreme event hydrographs, and mapped relative peak size using GIS. Hydrographs showed that runoff in the core area featured a steep rise and fall as well as extreme discharge measured as the ratio of June 2022 peaks to maximum historic annual peaks. Mapping this ratio showed that the spatial extent of extreme flows included northern YNP, the Absaroka-Beartooth Wilderness, and adjacent areas. Extremes were concentrated in the upper Yellowstone River Watershed, but also occurred in neighboring watersheds. The extent of extreme runoff was consistent with the type of narrowly focused Pacific Northwest atmospheric river storms intense enough to penetrate this far into the interior western mountains.