In June 2022 an atmospheric river produced historic flooding throughout rivers in northern Yellowstone National Park. The flooding caused substantial damage to roads and park infrastructure in the park and in surrounding communities. This award provided the opportunity to document the size and geomorphic impacts of flooding in the park, analyze hydrologic and landscape variables that contributed to variations in the style and impacts of flooding, and compare the June 2022 event to major floods in the last few hundred years. We documented the impacts of flooding based on field surveys performed in the summer of 2022 and 2023, in which we measured high water marks and channel geometry on 20 stream reaches from 12 ungauged rivers and streams across northern Yellowstone. This allowed detailed reconstruction of the size and variability of flooding across many of the rivers in northern Yellowstone National Park. Detailed impacts of the flooding, including bank erosion, channel scour, sediment characteristics, and bar deposition, were documented on 6 rivers. We complemented field-based analyses with remote sensing, including comparison of high-resolution pre- and post-flood topography data, and hydrodynamic modeling to reconstruct flood hydraulics.

Analysis of peak flooding variability indicates both geomorphic and meteorologic controls on the degree of flooding and spatial variability in erosion. The degree and impact of flooding differs between the Gallatin Range in western Yellowstone and the Beartooth-Absoraka Ranges in central and eastern Yellowstone, as a function of both precipitation and snowmelt amounts (the greatest precipitation and snowmelt occurred in high elevations of the Beartooth Range) and geologic differences that influenced infiltration and runoff dynamics and landscape susceptibility or resistance to erosion. Moving from west to east across northern Yellowstone, the flood magnitudes (normalized by area) increased, although geomorphic responses were more variable as a function of features such as valley confinement. The Gallatin Range’s bedrock geology is dominated by highly fractured bedrock, extensive thick soils, and gentler range flanks, allowing greater infiltration during the flood. On the western Gallatin Range flank, little bank erosion or bed material transport occurred where the Gallatin River flows on coarse channel bed material; reaches on finer bed material showed minor channel scour and gravel bar deposition. On the eastern Gallatin Range flank, the Gardner River had minimal bank erosion on upper reaches, but farther downstream, in the Gardner River Canyon, extensive bank erosion, landslides, and sediment deposition occurred, where the steep, confined channel focused stream power along the valley margins.

In contrast, the Lamar River, which drains the Absoraka and Beartooth Ranges, in northwestern YNP, and experienced far greater flood magnitudes, normalized by area, than in the Gallatin Range. The Lamar River’s drainage is characterized by steeper slopes largely on low-permeability rocks, with more exposed bedrock and relief up to 900 m, promoting rapid runoff and extreme flooding. In this region of YNP, the flood the produced an impressive hydrogeomorphic response in localized regions, including bank erosion, floodplain deposition, and road washouts, but in some low-gradient, unconfined valleys, the flood’s passage left little imprint.

Anthropogenic climate change is likely to increase the probability of extreme floods in YNP, as higher temperatures increase snowmelt rates, shift late-spring precipitation from snow to rain, and increase the propensity for extreme precipitation including that from atmospheric rivers. The 2022 flood, while massive in peak magnitude, was relatively short duration, as is more characteristic of the type of rain-on-snow events expected with climate change, compared to longer-duration snowmelt floods.

The research provided field geologic training opportunities for 10 undergraduate students from Whitman College and produced 3 senior theses, as well as supporting one M.S student at the University of Montana. The results were communicated to the scientific community and NPS personnel via three presentations at National meetings, meetings with Yellowstone NPS park staff, and a presentation at the Yellowstone Science Symposium. The causes of flooding, imprints of climate change on flooding, and hazard vulnerabilities to flooding in northern Yellowstone have been incorporated into case examples used in classes at Whitman College and the University of Montana and public presentations at Whitman College and with school and community groups in Montana. The award has led to continued collaborations between Whitman College, the University of Montana, and the National Park Service for ongoing work to perform a vulnerability assessment of the landforms and infrastructure of the upper Yellowstone River to recent and future climate-driven events. 

Last Modified: 12/04/2024
Modified by: Lyman P Persico