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In June 2022, a rain-on-snow (ROS) event occurred as the result of an atmospheric river delivering 2.5-10 cm of rain to the melting snowpack in and around Yellowstone National Park (YNP). The ROS event led to extreme flooding in northern YNP and caused extensive bank erosion, overbank deposition, and damage to infrastructure including roads and campgrounds. The degree of flooding and amount of erosion varied dramatically across northern YNP. To better understand the causes of spatial variability of flooding and erosion, we used cm-scale RTK GPS to survey flood stage indicators and channel dimensions. We typically surveyed 7 to 12 cross sections and the thalweg for each reach. A variety of stage indicators were identified including mud lines, debris lines, wash lines, and debris snags. We then used HEC-RAS to estimate peak discharge during the flood event. We estimated discharge on rivers draining the Beartooth Range (Rose Creek, Amphitheater Creek, Pebble Creek, Soda Butte Creek, Buffalo Creek, Cache Creek, and Slough Creek), the Washburn Range (Tower Creek, Lost Creek, and Blacktail Deer Creek), and the Gallatin Range (Gardner River and Gallatin River). We also determined basin area using StreamStats and calculated Riley’s Terrain Ruggedness Index from DEMs in ArcGIS. Drainage basin sizes for the surveyed reaches range from 2 km2 to 746 km2. Nearly all of the basins are categorized as moderately rugged, however, the basins draining the Beartooths are more rugged than those draining the Gallatin and Washburn Ranges. We found that, per drainage basin area, peak discharge was greatest in the Beartooth Range relative to the Gallatin and Washburn Ranges despite similar rain and snowmelt. The basins draining the Beartooth Range are characterized by extensive exposures of low porosity volcaniclastic bedrock and steep, glacially-sculpted drainages, which can generate significant overland flow, contributing to high peak discharge. In contrast, the Gallatin Range is composed of highly fractured sedimentary rocks and a thick cover of colluvium which facilitates a higher infiltration rate than the Beartooth Range, decreasing flood potential and resulting in lower peak discharge and less erosion. 

In June of 2022 an extreme atmospheric river flood caused extensive bank erosion and infrastructure damage in northern Yellowstone National Park (YNP). On the lower Lamar River, peak discharge was 170% of the next highest peak of 1996 (gaged since 1923) and resulted in widespread overbank gravel deposition and channel change. In June 1918, however, flooding on the Lamar system produced similar peak flows, as shown by indirect discharge estimates and tree-ring dating. In 2022, peak discharges and flood effects varied considerably in northern YNP. The upper Lamar River had a peak discharge significantly less than 1918, likely the result of less precipitation and snowmelt in the relatively low elevations of the upper Lamar drainage in the Absaroka Range. The high flows experienced by the lower Lamar River, however, were the result of extreme discharges in tributaries that drain the Beartooth Range where Soda Butte Creek and Pebble Creek had discharges similar or greater than 1918 and discharge on Slough Creek produced extensive mid channel bar deposition greater than 2 m. In western YNP, the Gallatin River experienced little bank erosion or bed material transport, although some reaches showed minor channel scour and gravel bar deposition on glacial outwash substrates. In central YNP, the Gardiner River experienced minimal bank erosion on upper reaches, however, there was extensive bank erosion, landslides, and sediment deposition in the Gardner River Canyon where the steep, confined channel focused stream power along the valley margins. Flood magnitudes differed markedly between the Gallatin and Beartooth drainages despite similar amounts of rainfall and snowmelt. The Gallatin River drainage, dominated by highly fractured and macroporous limestone and extensive thick colluvium with gentler range flanks allows for greater infiltration and reduced peak flows. In contrast, basins in the Beartooth Range drain steeper slopes of low-permeability laharic volcaniclastic rocks, with more exposed bedrock and relief up to 900 m, which promotes 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.