THE EXTREME 2022 ATMOSPHERIC RIVER FLOOD, CONTROLS ON FLOOD GENERATION, AND CLIMATE CHANGE IN NORTHERN YELLOWSTONE NATIONAL PARK

The June 2022 floods in northern Yellowstone National Park (YNP) have been described as a 500-year event, one that caused extensive bank erosion and damage to roads and campgrounds. An atmospheric river delivered 2.5-10 cm of rain to the high-elevation snowpack around northern YNP from June 10-13. On the Lamar River, a major Yellowstone River tributary in NE YNP, the peak discharge of 950 m³/s was the flood of record, 1.7 times the next highest peak of 1996 gaged since 1923; widespread overbank gravel deposition and channel change occurred. In June 1918, however, extreme flooding on the Lamar system produced similar peak flows, as shown by indirect discharge estimates and tree-ring dating, and was also associated with rapid warming, rain on snow, and major road damage. Overbank gravels provide evidence for comparable extreme floods ~1873 and possibly ~1790. In 2022, peak discharges and flood effects varied considerably over northern YNP. On the western Gallatin Range flank, little bank erosion or bed material transport occurred where the Gallatin River flows on coarse till and gravel; reaches on finer glacial outwash substrates showed minor channel scour and gravel bar deposition. Peak discharge on the lower Gallatin was 94% of the flood of record. On the eastern Gallatin Range flank, the Gardner River peak discharge was 120% of the record, with minimal bank erosion on upper reaches. Extensive bank erosion, landslides, and sediment deposition occurred in the Gardner River Canyon, however, where the steep, confined channel focused stream power along the valley margins. Flood magnitudes differed markedly between the Gallatin and Absaroka-Beartooth drainages, despite similar precipitation and snowmelt. The Gallatin Range is dominated by highly fractured and macroporous limestone, extensive thick colluvium and talus, and gentler range flanks, allowing greater infiltration. In contrast, the Lamar River drains steeper slopes largely on erodible, low-permeability laharic volcaniclastic rocks, with more exposed bedrock and relief up to 900 m, promoting rapid runoff and extreme flooding. The frequency and magnitude of rain-on-snow floods is likely increasing in YNP because of anthropogenic warming as the high-elevation snowpack becomes more susceptible to rapid melting and late spring precipitation shifts from snow to rain.