Yellowstone flooding: Why is it happening now?

Rain falling on snow caused this week’s floods, events that will become more likely as temperatures warm.

Yellowstone National Park evacuated more than 10,000 visitors on Tuesday after flash floods roared through the park. Roads and bridges washed away, sewage lines broke, and the park’s gateway communities were cut off from roads. Yellowstone remains closed, and the north entrance gate will likely not reopen this season.

Even as scientists and land managers are taken aback by the magnitude of the floods, unprecedented in 100 years of recorded history, they recognize the similarities to the events their data predicted. They just weren’t anticipating them to occur this year.

“As a scientist I would say, well, this is completely in line with what we might expect,” says Cathy Whitlock, a paleoclimatologist and lead author of the Greater Yellowstone Climate Assessment, the first such report ever done on an ecosystem. “As a human being I would say I'm shocked.”

While it will take more research to confirm if climate change made this flood event more extreme, the 2021 Greater Yellowstone Climate Assessment predicts significant changes in precipitation, including when it arrives and what form it will take. Scientists expect more spring rain, less winter snow. The assessment also predicts increasing annual precipitation in Yellowstone National Park.

Changes are already documented in the assessment. Since 1950, spring precipitation in the region has increased by 17 percent in April and 23 percent in May. Snowfall has declined even as overall annual precipitation has increased. That means rather than a slow release of melt water to valleys during the summer months, rainfall tends to combine with melting snow, causing riskier events like the recent flood.

Changing climate shapes the landscape

Throughout its tumultuous geologic history, climate change and extreme flooding shaped the landscape around Yellowstone. Even today, shifting natural events are part of life in the western United States. For example, last year, Yellowstone National Park received the lowest precipitation recorded in June—ever.

Ann Rodman,Yellowstone National Park’s Geographic Information Systems manager, who works on climate adaptation planningthought this summer would be similarly parched. Just two months ago, she calculated a 5 percent chance of flooding in the park. Then a big snowstorm fell in the mountains over Memorial Day after an already cool and wet spring.

Still, she expected those mountain moisture gains to dry up once the hot months started.

In mid-June, the jet stream carried in exceptionally wet weather dropping rain on the unusually snowy mountains. Warm temperatures helped melt the snowpack. And as temperatures warm, these rain-on-snow events are projected to become more frequent at higher elevations in western North America, while lower snowpack will make them less frequent at low elevations.

 Yellowstone’s Superintendent Cam Sholly said during a Tuesday press conference he’s been advised that the flood could be a thousand-year event. One of the highest recorded stream flow measurements for the Yellowstone River was 31,000 cubic feet per second, recorded in the 1990s. Stream flow readings during the recent flood were as high as 51,000 cubic feet per second.

“What happened this week was not anything I was expecting,” says Rodman. June precipitation this year is now at more than 400 percent above average in the parts of Montana and Wyoming that include Yellowstone. “We had rain that went all night and all day and it was kind of torrential. We just don’t get that kind of rain here.” On Monday, she watched from her house in Gardiner, just outside Yellowstone’s north entrance, as park employee housing slid into the Yellowstone River.

While the current floods are similar to what Rodman and others expect based on long-term climate predictions, they are surprising in the sense that just earlier this month she was ready to talk to people about severe drought and fire.

“We agree that we couldn’t have anticipated this,” says Rodman. “So what should we learn from this and what should we be thinking about going forward?”

Preparing for the worst 

That growing need to expect rapidly changing conditions is why Yellowstone National Park and the National Park Service are looking ahead to plan for different scenarios as the climate changes. Bruce Stein, chief scientist of the National Wildlife Federation, collaborated with the Park Service on a 2021 climate planning report. He says relative to other federal agencies the organization is “ahead of the curve.”

But he too was caught off guard. Several years ago, when he trained Yellowstone National Park employees for climate planning, they focused on the features that draw visitors to Yellowstone—like bison and wolves. Roads, bridges, and houses getting washed away were not on their radar.

“Even when we try and do our best to project the possible range of things, we end up being surprised,” says Stein. “And I think that the flooding in the Yellowstone kind of falls into that category.”

“It’s those unexpected and oftentimes worse than the worst case projected scenarios that we are starting to see.”

Thanks to the recent floods, Yellowstone National Park and small towns in southwest Montana will join the ranks of communities that are being forced to rethink their infrastructure, or likely end up rebuilding it after the next flood.

“It's really clear that our infrastructure is not well placed for climate change,” says Whitlock. “The fact that we're losing these roads, and there's these massive landslides and houses are washing away clearly means that we’re not thinking enough about the impacts of climate change in our land use and in our infrastructure development. And we need to do a better job of that. Because that's the cost and that's the tragedy.”

The road ahead

Yellowstone National Park is already taking a cue from climate planning in thinking about the now-washed out  north entrance road that used to carry visitors the five miles from the park’s popular north entrance at Gardiner to its headquarters at Mammoth Hot Springs. The park superintendent said at the press conference that the road will not be built back in the same place, citing changes on the horizon.

“I think it is smart and forward looking,” says Rodman. “[The superintendent] is very aware of the need to consider potential impacts from climate change when planning infrastructure projects. I think the recent event underscored just how big those impacts can be.”

Similarly, Denali National Park in Alaska is battling road closures caused by melting permafrost, and is exploring alternatives to road repair, including a bridge over the landslide or re-routing the road.

As climate planners, Rodman and her colleagues struggle with these issues daily. “It affects everything, so how do you get a thoughtful way with limited resources? What's the most effective thing you can do that will actually make a difference?”

For the millions of people who love national parks and Yellowstone, and for those who live in the region, or had their summer plans dashed, unprecedented events can come with a kind of grief, or even resentment.

Stein suggests the public try to embrace the unexpected, even as it can be heartbreaking and difficult to watch our favorite places rapidly and sometimes dramatically reshaped. 

“I think at a deeper level to understand and love the concept of what a national natural park is, is to accept that there are changes that have always been taking place,” he says.

“And, of course, now they’re taking place on an accelerated scale. What makes it difficult is that so many of these accelerated changes are due to climate impacts that have a human fingerprint on them.”

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