Yellowstone Area Floods of June 2022: Flood Potential Perspective

The recent Yellowstone area floods have been reported in the media as unprecedented and induced by climate change. The flood potential method, developed by the Forest Service National Stream and Aquatic Ecology Center, provides techniques to evaluate such claims.

Without a doubt, the impacts of these floods have been catastrophic to the communities and infrastructure in the path of the floodwaters. Inundation and fluvial geomorphic adjustment (erosion and sediment deposition) was extensive, as is expected when large floods occur in such landscapes. This resulted in the loss of stream valley infrastructure, homes, businesses, and property. Preliminary data collected by the U.S. Geological Survey (USGS) indicate that these flood magnitudes were unprecedented (record values) at several streamgages. Considering this, should this event be considered unprecedented from a larger perspective?

The flood potential method provides a systematic procedure to quantify, compare, and communicate about flooding. Applying this tool to the Yellowstone area event indicates a number of things:

• These floods were large, and in some locations station record setting and extreme. But this event is not unprecedented considering the history of recorded large floods in this area (from 1906 to present). Generally, this event appears to be towards the high end of the experienced variability, with extreme flooding in some areas but a more extreme flood experienced in 1945, and with similar-scale extreme floods (to 2022) experienced in 1955, 1963, 1981, and 1996.
• While this flood may have not been unprecedentedly extreme, the large spatial extent of the impacted area may be unusual. This is reminiscent of the Colorado Front Range Flood of 2013.
• Within this zone, large floods most frequently occur in June (see figure below). This event occurred during the month when it is most expected.
• The 2022 event supports a previous finding that large floods are possibly increasing in magnitude in this area. Increases in design flood discharges of 7% from base values have been previously recommended, due to the increasing trend. The recommended adjustment is expected to increase due to this event.

Hence, it can be reasonably stated that while this flood has been extreme in places and catastrophic for local communities and our transportation infrastructure,?given the available preliminary data this event appears to not be unprecedented. Similar- and larger-scale extreme floods have previously occurred, with these 2022 flood magnitudes being within an upper range of large flood variability that occurs in the area. Such events are part of the larger-scale mode in a bimodal flood regime.

Large floods have been becoming larger in magnitude by 7% in this area. Inclusion of the 2022 data in an updated analyses is expected to increase this observed trend in the magnitude of large floods. Due to the lack of other known mechanisms for explaining this increase, the increasing flood magnitudes are likely due to climate change. For context, 29% of the western United States (between the Pacific Ocean and the Mississippi River) is currently experiencing increasing trends in the magnitude or frequency of large floods.

We look forward to revising the flood potential analyses for this zone once the USGS has had the opportunity to finalize peak discharge values. Expected flood potential (Qefp) and maximum likely flood potential (Qmlf) discharges, combined with flood magnitude correction values to account for an increasing trend, are expected to be valuable for use in the selection of new design flood discharges (alongside the results of updated flood-frequency analyses). These updated discharge estimates will be essential for replacing infrastructure and developing communities that are more resilient to such floods when they next occur.

Below are the (preliminary) streamgage data and other information used to make these conclusions:

• The flooding primary occurred in flood potential zone 46. The flood potential plot for this zone (using pre-2022 data) is shown here, with average annual precipitation (P) being a significant second predictor.

• The most extreme flood to have occurred in zone 46 had a flood extreme index (Ef) = 1.76 in June of 1945.?Since the following streamgages have Ef values less than this, we can state that this 2022 event (while large and devastating, and in some locations extreme) is not unprecedented in magnitudes for this zone.
• Yellowstone River near Livingston, MT (ID: 06192500); 55,400 cfs on 6/13 @ 23:00; flood extreme index = Ef = 1.31 (not extreme)
• Yellowstone River at Corwin Springs MT (06191500); 49,400 cfs on 6/13 @ 1500; Ef = 1.47 (extreme)
• Lamar River nr Tower Ranger Station YNP (06188000); 20,900 cfs on 6/13 @ 0730; Ef = 1.59 (extreme)
• Yellowstone River at Yellowstone Lk Outlet YNP (06186500); 4580 cfs on 6/15 @ 0915 (still slowly rising); Ef = 0.28 (not extreme)
• Boulder River at Big Timber MT (06200000); 11,000 cfs on 6/13 @ 1845; Ef = 0.99 (not extreme)
• Stillwater River near Absarokee MT (06205000); 23,900 cfs 6/13 @ 1245; Ef = 1.44 (extreme)
• Rock Creek near Red Lodge MT (06209500); >3000 cfs (gage inundated); Ef > 0.85 (unknown)
• Clarks Fork Yellowstone River nr Belfry MT (06207500); 23,900 cfs on 6/13 @ 1415; Ef = 1.21 (not extreme)