[Note that this article is a transcript of the video embedded above.]

Every year, a deluge of tourists stream into Yellowstone National Park, America’s first and possibly most famous national park, and (I would argue) one of the most beautiful and geographically rich places on earth. But this past June of 2022, many of those tourists, along with some of the permanent residents of the area, found themselves at ground zero of a natural disaster. Torrential rainfall in Wyoming and Montana brought widespread flooding to the streams and rivers that flow through this treasured landscape and beyond. Homes, bridges, roadways, and utilities were swept away and over 10,000 people were evacuated. As of this video’s production, the National Park Service is still picking up the pieces and deciding how to restore the damaged infrastructure within the park, but while the NPS is busy with that monumental task, I wanted to share the engineering details we already know about what happened during the flood, how they might rebuild the roads and bridges stronger than before, and why they might not want to. I’m Grady and this is Practical Engineering. In today’s episode, we’re talking about the 2022 Montana and Yellowstone floods.

If you didn’t grow up with posters of the Old Faithful geyser on your classroom walls and watching Yogi Bear raiding picinic baskets, that’s okay. I can give you a quick tour. Yellowstone National Park celebrates its 150th birthday this year since it was established in March of 1872. The park covers the northwestern corner of Wyoming and extends into Montana to the north and Idaho to the west. It’s a big place, roughly half the area of Wales, if that’s a helpful equivalency for those more familiar with the metric system. And there really is a lot to see. There are geysers here, here, and here where hot water and steam are ejected from the earth at regular or irregular intervals. In fact half of the world’s geysers are located in the park. There are hot springs, vents, and mudpots here, here, and here . There is a massive natural lake that freezes over each winter here. Waterfalls here and here. Plus mountains, valleys, wolves, bears, bison, and lots more spread throughout the entire park.

A series of roadways connects the five park entrances to the various attractions, lodges, campsites, and of course, their respective parking lots. Indeed, for better or for worse, the park service estimates that 98% of visitors never get more than a half mile away from their car. We bucked that trend during our visit in 2019, but only for a single hike. Otherwise we stayed on the beaten path along with the roughly 3 to 4 million other visitors per year that cram into the same 1% of the park’s total area.

Here’s why that’s important to the story: Many of the most visited areas of Yellowstone are along the rivers and streams that run through the park, largely due to the unmistakable beauty of those rivers and streams as they flow into and over the striking geologic features. However, that proximity of development to the watercourses in the park became a serious and nearly deadly complication this June. On the night of the 12th into the morning of the 13th, an enormous storm system dropped rain across nearly the entire Yellowstone area and large parts of Montana to the north. Some areas saw more than 4 inches or 100 millimeters of rain in less than 24 hours. What’s worse is that a lot of those inches and millimeters fell on top of snow-covered ground, rapidly melting the snowpack and exacerbating runoff. These so-called “rain-on-snow” events have a long history of contributing to floods, and the 2017 Oroville Dam spillway failure that I’ve also covered on the channel was partly a result of rain-on-snow flooding.

All this rain and snowmelt concentrated in the streams and rivers that flow through the park. The US Geological Survey has several stream gages spread throughout the park and southern Montana, so we can take a look at the data to see exactly what happened. And the National Park Service posted an album of aerial photos on their Flikr page so we can compare the streamflow records to the damage on the ground.

A few places on the edge of the storm only saw a small spike in streamflow. For example, the Firehole River that carries water from Old Faithful only went up by about a foot and a half (or 45 centimeters). That river comes together with the Gibbon River along the West Entrance Road, where, again, the increase in streamflow wasn’t overwhelming. But near the northern border of the park, things were much more serious. The river in the Lamar Valley, sometimes called America’s Serengeti for the huge populations of bison and other large animals, came up nearly 9 feet or about 3 meters, briefly surpassing the “moderate flood” stage, which is the level at which the National Weather Service expects damage to buildings and infrastructure to begin. At locations where the valley narrows, the torrent of water eroded and destabilized the river bank, threatening, and in some cases destroying the adjacent roadway. The Soda Butte Picnic Area was hit the hardest in this part of the park.

The Gardner River at the north entrance of the park came up about 2 feet (60 centimeters) at the stream gage, but that number doesn’t quite capture the devastation. A good portion of the flood damage in the park happened along a single stretch of road where the Gardner River created massive washouts and rockslides. In many places, the entire road has been completely washed away where the river altered its course to flow through where the road once was.

Many of these streams confluence into the Yellowstone River that flows through southern Montana, and flooding continued along this river out of the park. One employee housing structure fell completely into the river and floated away. The USGS estimated that the Yellowstone River exceeded the 500 year flood stage nearly all the way to Billings, wreaking havoc on the communities along the river. I’ve talked about this “blank-year” flood in a previous video, but I’ll explain it briefly here. Engineers can look at historical data to estimate a relationship between a flood’s magnitude and its likelihood of happening in a given year. The 500-year flood is just a point on this line. Obviously this is not an exact science (for a bunch of reasons), but it’s helpful for engineers, actuaries, and planners to think of flood magnitude in terms of its probability. Even though the name implies it can only happen once every five hundred years, the actual definition is a flood magnitude with a 0.2% percent chance of being exceeded in a given year.

With this widespread and tremendous flooding, more than 10,000 people were evacuated from Yellowstone National Park. Although the National Weather Service had rain in the forecast, there was no expectation of such significant rainfall, forcing employees to scramble overnight to close roads and get people out of harm’s way. Remarkably, not a single person was injured or killed in Yellowstone as a result of the flooding. Also incredibly, on July 2 (only two-and-a-half weeks after the flood occurred), the park announced the north loop was back open to vehicular traffic. As of this video, the only major parts of Yellowstone that are still closed are the two northern entrances and their respective roads leading into the park. This is due in large part to the fact that there were already roadway contractors working on other projects when the floods happened. We don’t have all the details yet, but it’s likely the Federal Highway Administration was able to amend one of those contracts to get help repairing some of the flood damages expeditiously.

Speaking of those damages, we still don’t know their full extent. The Park Service has a lot of work ahead of them to inspect the condition of backcountry bridges, trails, campsites, and park infrastructure. Over $60 million dollars in “quick release” emergency funds have already been released to help with emergency repairs, and some news agencies have speculated that the total repairs will cost up to a billion dollars based on costs of similar repair projects at national parks.

The highest priority repairs will be those along the northern entrances to the park where the rivers changed their courses into roadways. It’s not just the park that is affected by those closures but the communities outside the park that depend on seasonal tourism. Damage in these areas will also be the most challenging and difficult repairs to complete, likely requiring completely new roadway alignments that will come with environmental and archaeological studies, public feedback, permits, geotechnical studies, and careful design all before construction begins.
As an example, the Yellowstone River Bridge replacement project started planning and design in 2019 and was set to start construction this year until floods delayed the project, so that’s a roughly 4-year pre-construction phase. Some people might call this unnecessary bureaucracy and red tape, and certainly the communities that depend on Yellowstone traffic will be hoping for much speedier temporary repairs to these roadways. But, many might also consider this careful planning and design as good stewardship for one of the most beautiful places on earth. Hasty engineering of large infrastructure can be extremely damaging to natural systems like those in Yellowstone, and you don’t want to invest millions of dollars into repairs that might be subject to similar flooding in the future. After all, we build parks (and roads to parks) to get closer to the natural environment and all its wildness, and there’s almost nothing more natural or wild than a flood.