Flooding could breathe life into Yellowstone ecosystem
Although destructive for people, high-water events are a natural part of river systems.
Spring rains atop deep snowpack fueled mid-June floods that inundated hundreds of homes, forced more than ten thousand tourists to evacuate, and caused millions of dollars’ worth of damage in the Yellowstone region, especially in southern Montana. Weeks later, the effects on the ecosystem were just beginning to emerge.
A week and a half after the historic floods, I walked down a gravel road in Montana’s Paradise Valley with geomorphologist Karin Boyd, who runs a private consultancy in Bozeman focused on restoring river systems. We entered a normally bustling fishing-access site, closed due to the flooding. With no anglers or campers, it felt like a ghost town. The Yellowstone River was still high and muddy, around 30% above the average flow for this time of year.
“You don’t want to come out here and celebrate when people are hurting,” Boyd said. But, she said, there is a lot to celebrate. Big floods — even if devastating to human communities — can help ecosystems like this one thrive.
As Boyd and I walked, we could see evidence of both destruction and regeneration. A waterlogged roof truss bobbed next to the bank. Caramel-color sediment, cracked like alligator skin, covered the top of a picnic table — an indication of how high the floodwaters had risen. Nearby, otter and beaver footprints dotted the ground. A red-tailed hawk soared overhead, and mergansers drifted down the turbid water.
A little giddy, Boyd pointed at depressions in the earth, still saturated with water, where riparian vegetation might be seeding. “Willows and cottonwoods, man, those are just incredible in systems that get disturbed,” she said. Cottonwoods release seeds during a narrow window each year, just after peak runoff. They need flood-scoured areas, where rushing water has deposited nutrient-rich soil and distributed seeds, to reproduce. Floods like this one can be crucial to creating and maintaining lush, diverse riverside habitat.
But the flood’s impacts aren’t all visible yet, Boyd said, and not all of them are positive. The success of those cottonwood seeds depends on the water receding gradually in the weeks to come; a sudden drop could leave the water table too low for their roots to reach. Meanwhile, the same water that deposited those seeds could also have carried their competitors — weeds and invasive plants — into new areas.
At the same time, the region is facing other, much greater challenges — mainly climate change. An assessment of the Greater Yellowstone Ecosystem speculates that the region could warm another 5 to 10 degrees Fahrenheit by 2100, meaning more drought, more floods and more wildfire.
And yet, Boyd says, despite everything, she’s optimistic about the future here.
THE YELLOWSTONE is the longest major undammed river in the Lower 48 and part of one of the largest river systems in the world. It’s also a “Blue Ribbon” river, known for its world-class trout fishing. Anglers in the area generally target rainbows, brown trout and Yellowstone cutthroats.
“Floods are horrible and devastating to humans and infrastructure when (the water) comes out of those riverbanks,” said Scott Opitz, a fisheries biologist with Montana Fish, Wildlife and Parks. “It’s almost the opposite for fish.”
Opitz explained that all trout have adapted to flooding to some extent. They generally hang out in slack water near the riverbanks and rise with the water. Surging water deposits woody debris that can provide new safe havens for fish throughout the river. Floods also move and clean gravel and cobblestone, creating ideal new spawning grounds.
Back at the closed fishing-access site, I’d seen these processes in action. The floodwaters had eaten away at the outside edge of a horseshoe-shaped bend in the river, gashing the earth within a couple feet of the road. Even over a week after the flood, cobblestones and gravel trickled — sometimes cascaded — from the incision into the river below. Elsewhere, bare cottonwood roots fanned out over deep pools near the riverbank — prime trout habitat.
Opitz said the flood could impact different species in different ways. Browns and rainbows, for example, were both introduced by anglers over a century ago and aren’t adapted to the region’s cycle of runoff; brown trout spawn in the fall and rainbows generally reproduce in the spring, just before peak flows. Opitz speculated that the high, turbulent water could hurt both species’ reproduction.
Native Yellowstone cutthroat, he said, are a different story. Their populations have been struggling in many areas, in large part due to warming waters from climate change and competition from nonnatives like browns and rainbows. But they’ve evolved to tolerate large-scale floods like this one. Cutthroats spawn after peak flows, as the water begins to recede. By rejuvenating habitat and gravel beds in which to spawn, Opitz said, the flood might actually help Yellowstone cutthroats thrive.
BIG FLOODS CREATE habitat for more than just fish. For more than 40 years, Ric Hauer, professor emeritus of systems ecology at the University of Montana, has studied gravel-bottomed rivers like the Yellowstone. “These are the major sites of biological activity and biodiversity — for birds, for ungulates, for wolves chasing them,” Hauer said. “Bears, eagles. Everything is focused on the floodplain.”
Hauer’s research shows that about 70% of the bird species in the Yellowstone area depend on streamside, or riparian, habitats in floodplains — the habitats created by those cottonwoods. Smaller creatures, too, rely on the dynamics of flooding rivers. As floodwater rushes downstream, ripping up sediment in some places and depositing it elsewhere, water filters through the river’s gravel bottom and back up again. Microbes living beneath the surface attack organic matter in the river, releasing nitrogen and phosphorus, creating what Hauer calls “hot spots for more productivity,” where algae flourish. Aquatic insects feed on that algae, and fish in turn feed on those insects.
“It’s an amazing, amazing system,” Hauer said.
A few days after meeting with geomorphologist Karin Boyd, I hiked the road that runs from Yellowstone National Park’s North Entrance, along the Gardner River. It was still closed to vehicles, and with no cars or people in sight, the park felt eerily silent. Salmonflies the size of my pinky finger perched lazily on tall grasses. Elk tracks dotted the still-wet floodplain. White, fuzzy puffs of cottonwood seeds floated in the air. Eventually, I arrived at an area where the river had reclaimed the roadway: The pavement ended in a sharp plunge to flowing water about 30 feet below. Marveling at the floodplain, I almost stepped over the road’s jagged edge.
The juxtaposition of the human and natural landscape here reminded me of something I’d discussed with Boyd. “In the Yellowstone, we’re so blessed with this,” she’d said, gesturing to the flowing, muddy water and the distant cottonwoods. She told me that she’s worked in areas where rivers and creeks have been confined, channelized and generally controlled in the name of protecting human infrastructure and development. Now, as plans to rebuild take shape, people in the Yellowstone area are figuring out what comes next.
Nick Mott is an award-winning journalist and podcast producer who focuses mostly on climate, public land and the environment. He’s based in Livingston, Montana.
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