When spring rains and melting snowpack rushed down the mountains of southwest Montana in mid-June, flooding the Yellowstone River and its headwaters tributaries, I studied photos of beloved small towns inundated with chocolate-colored water, and checked on evacuating friends. I thought of the upended lives and livelihoods, and of the washed-out roads and other damaged infrastructure that will take years and millions of dollars to repair.
Then I thought about a solitary cottonwood tree in Yellowstone National Park’s Lamar Valley.
One recent August, while fishing for Yellowstone cutthroat trout along the rock-bottomed Lamar River, I looked for a shady lunch spot to escape the baking sun. While the Lamar Valley is known for attracting bison, gray wolves, and grizzly bears, the river-side plants supporting their habitat have been in decline. There were few shade trees nearby—and most had easily-disgruntled bison under them.
I finally settled under one giant, furrowed cottonwood, likely at least 100 years old, perched well above the riverbank. This loner was a hanger-on from a long ago flood that spread new moist soil on the valley floor, creating the precise conditions cottonwood seeds need to sprout. No young cottonwoods grew nearby. I’ve met trees like that Lamar Valley cottonwood all over the western U.S., and wondered which among them are the last to grow in those places.
Between severe drought, climate change, and water diversion and control, opportunities for rivers to spill into their floodplains are much rarer than they used to be in the contiguous Lower 48 states. That has many consequences. Channels erode and deepen, banks are choked by invasive species, and wetlands that naturally store water are lost.
So when historic flooding hit Yellowstone National Park, I wanted to know if it could have a restorative effect on the ecosystem. Whether or not there is scientific justification for optimism matters far beyond the prospect for future cottonwood groves that I’d all but given up on. At stake is the future of an iconic and culturally important landscape.
Floods reshape landscapes
As they flood, wild, undammed rivers like the Yellowstone and its tributaries do something humans tend to resist—they change. In wild rivers, fast moving water seeks out new channels, carries soil and gravel downstream, and spreads it across floodplains, reshaping the terrain. What may look like destruction to an angler used to casting a fly from a favorite sandbar, a boater accustomed to taking a particular side channel, or a landowner whose property line shifted, is in fact a sign that a river and its floodplain are vibrant and alive.
“As humans, we often think that floods are disastrous, and fires are disastrous, but they're really only disastrous because we put human lives and property in harm's way,” says Scott Bosse, the director of American River’s Northern Rockies office. “They're not disastrous from an ecological standpoint. Quite the contrary, they're extremely healthy for rivers, and especially for a river like the Yellowstone.”
Karin Boyd, a Bozeman-based geologist who studies how rivers are shaped, helps people understand how they can learn to work with rather than against rivers. She has been eager to see what landscape changes the Yellowstone River’s muddy waters reveal as they recede.
“We always think about erosion damaging things,” she says. “But what it does is just recruit hundreds of thousands of cubic yards of sediment into the river, and that stuff is going to be reworking and readjusting with time as the river kind of settles down. And that's something I'm just super interested to see.”
She’ll be looking for features like downed trees snarled up in the river channel and on banks. Over time, as those piles capture sediment they grow into sand bars or islands. Eventually, willows and cottonwoods can gain a toehold on that freshly built land and create new animal habitat.
An opportunity for trout
Native trout like Yellowstone cutthroat also favor the kind of complexity floods create. While waters are raging, the trout escape to calmer side channels. As higher water makes tributaries more accessible, they use them to spawn. Plus, trout don’t stop feeding during floods.
“During the event itself, there's a tremendous amount of scour, which means there's a ton of drifting invertebrates in that high water, just providing a buffet line for fish and aquatic creatures,” says Pat Byorth, who spent 17 years as a state fisheries biologist in Montana, and now directs Trout Unlimited’s Montana Water Project.
Native trout have evolved to thrive in flood-prone rivers over the roughly 150,000 years they’ve been on this landscape. Introduced species, like rainbow trout, have not. More of their eggs or newly emerged fry may have been scoured away by the flood.
Byorth expects a strong spawning population of Yellowstone cutthroat trout in three years, at which point anglers will enjoy catching them, similar to the years following floods in 2011 and 1996. Meanwhile, biologists may not count as many rainbow trout in future years, though he doesn’t expect it will be a large enough change that anglers will notice.
Byorth loves to fish, he says, but when he thinks about the benefits of flooding he’s considering insects like stoneflies, whose river-rock habitat will benefit from upheaval, and birds like the American Dipper that eat those aquatic insects. “It affects osprey, eagles, river otters,” says Byorth. “Everything in our ecosystem, and especially in the park, relies on this kind of refreshment to propagate itself in the long-term.
“This is a complete remodel of the aquatic ecosystem and it's a beautiful thing.”
Cottonwoods are seeded by floods
One of the big ecological questions about this flood is whether and how it will boost riverside vegetation, particularly cottonwood and willow trees. Globally, riparian habitats are biodiversity havens; in the western U.S, they support more breeding birds than all other regional habitats combined. Since cottonwoods are the dominant riverside tree, their fate has ripple effects throughout the ecosystem.
To regenerate, both cottonwoods and willows first need to release their seeds onto wet, sandy soil, which usually comes from flooding or beaver dam breaches. Those conditions haven’t existed at a large scale in northeast Yellowstone National Park in the last 25 years—but this year’s floods alone may not rectify the situation.
Many of the cottonwoods in northeast Yellowstone today took root during a series of wet years in the 1990’s. After the floods of 1995 to 1997, 1.36 million of the 1.37 million cottonwoods along the Lamar and Gardner Rivers and Soda Butte Creek were established, according to a study by David Cooper, a Colorado State University ecologist. Multiple flood years in a row were key to those last million trees succeeding. If seeds germinate one year, they often don’t survive if they are left high and dry the next spring.
“Cottonwoods in particular are the most drought-sensitive trees in North America,” says Cooper. “They really have a very poor ability to deal with low soil-water content, and so they need to have a steady supply of water.”
There also need to be enough small, fuzzy cottonwood seeds falling on the moist soil. Anyone who has ever parked a car under a cottonwood tree knows about this “seed rain.” But Cooper is concerned that plants in northeastern Yellowstone have been browsed too heavily by elk and bison for there to be enough seeds this year—and also that any young trees that do get established might be mowed down by bison or elk. That happened to many cottonwoods that started growing after the 1995 to 1997 floods.
“If you do get things established, are there places where they won't get massacred?” Cooper says. “It’s really a challenging place.”
New vegetation patterns may also depend on the kind of valley surface the receding floodwaters leave behind. In the past, large floods going back to the late 1800s and early 1900s left gravel deposits on the valley floor of a Lamar River tributary, according to research by Grant Meyer, a geomorphologist at the University of New Mexico. Those fast-draining gravels replaced the fine, water-retaining sandy soil favored by grasses and cottonwoods. Instead, lodgepole pines moved in.
Over the long term, flood severity or herbivores aren’t Meyer’s greatest concerns for Yellowstone’s riparian habitat. “It's simply climate change, and we're going to get extreme droughts too, there's no question about that,” says Meyer. “Of course we've had them more so than flooding, so that's going to hinder [cottonwood] regeneration more than anything.”
Yet many researchers and conservationists remain optimistic that this year’s floods will lead to renewal along the rivers in and around Yellowstone.
“We're going to have an amazing year for cottonwood regeneration next year, because cottonwood needs fresh sand beaches to germinate,” says Bosse. “We're going to have an entirely new river on the Yellowstone next year, and it will be fascinating to explore.”
Boyd is also anticipating regrowth. After massive 2011 floods on eastern Montana’s Musselshell River, she saw carpets of young cottonwoods spring up. There, she worked on a program to compensate landowners who protected the tender young plants from grazing cattle.
“A lot of us are pretty bullish,” she says. “If you start seeing all of this sprouting, I think the park management folks will be thinking about, ‘Okay, do we need to protect some of them? Are they going to get trampled by bison, or grazed out?’”
Those grazing mammals, meanwhile, are having a fat spring. The rain and snow that caused the flooding is benefiting plants all over the landscape. Though elk migrating to their summer calving grounds in the park will have to cross swollen, potentially dangerous, rivers, all the animals in northern Yellowstone are enjoying one striking change: a respite from people and cars. While the northern part of the park remains closed, biologists expect animals to use the landscape in ways we don’t usually see.
Misplaced roads and buildings
In the long-term, advocates of unbound rivers see the recent floods as an opportunity to change our infrastructure so it’s less at risk and more beneficial to people and the ecosystem. The rain on snowfall events and extreme weather that caused this year’s flood are likely to occur more frequently as the climate changes. With that in mind, park officials have discussed moving the severely damaged road along the Gardner River to higher ground. Not only will that prevent future wash-outs, it creates another opportunity for a river to take back its floodplain.
“I think the greatest threat is overreacting and building hard infrastructure … in ways that reverse the benefits of the large flood to the aquatic ecosystem,” says Byorth, whose own family suffered property damage from the flood. He says it very likely would have been worse without the stream-side vegetation growing in front of their cabin.
“Yeah, we have to build roads, we have to build bridges,” he says. “But we don't have to go in and re-channelize streams and put them back in boxes and concrete chutes. We can learn from the flood and recraft our infrastructure in ways that let the river be wild and stay healthy over the longer term.”
However that infrastructure is rebuilt, it will take years. For now, the northern part of the park, including the Lamar Valley, remains closed.
Like most people, I didn’t expect to see it in person again this year. Instead of fishing one of the Lamar’s tributaries on my June birthday as planned, I spent that day at a press briefing that detailed Yellowstone’s damage. At the first washout only minutes from the park’s north entrance gate, I stood with a gaggle of reporters behind yellow cones, staring at the new bank the Gardner River had carved as it swallowed the road. Under a thin stratum of asphalt and a broken sewer pipe, the deep layers of smooth cobbles and silt reminded us that the river had been in that spot many times before it was paved.
Then, just over a week after the peak of the flood, Chris Boyer, a Bozeman-based aerial photographer, farmer, and river restorationist told me he had an empty seat for a flight over the park. After he made an early morning delivery of goats elsewhere in the county, we climbed into his lovingly maintained 1957 Cessna, and crossed the Gallatin Range and followed the Yellowstone River into the park. As we approached the Lamar Valley, we could immediately see the flood’s work: the freshly exposed soil and gravel, the stacks of washed-up trees, the eerily empty road along the river. We were too high for me to tell if the old cottonwood whose shade I’d appreciated had survived.
We turned into the mountain drainage I’d been planning to fish this month, and marveled at the creek’s many old horseshoe bends left behind as the river shifted course over time. Brown flood water now contrasted with bright green vegetation, highlighting the great curvatures on the land. “I’ve always thought the meandering pattern of a river is one of the most beautiful shapes in nature,” Boyer says into his headset. Mesmerized by the scene below, I had to agree.