Yellowstone Declares War on Rainbow Trout: What the Buffalo Creek Closure Means for the West's Most Important Fishery
Late August in the Lamar Valley is about as close to wilderness perfection as the American West still offers — wolf packs moving through sage flats at dawn, bison standing knee-deep in cold tributaries, and the kind of trout water that makes fly fishermen lose sleep the night before a trip. But starting August 31, one slice of that landscape is going dark for a week, and the reason speaks to a conservation battle that has been grinding on inside Yellowstone National Park for decades, one creek at a time.
Beginning August 31, Yellowstone National Park will temporarily close Buffalo Creek, a tributary of Slough Creek in the northern part of the park, for treatment to remove invasive fish as part of ongoing conservation efforts to preserve native species. The creek will reopen to the public on September 7. Eight days. A tight window, a precise mission, and a chemical tool that most people have never heard of — all aimed at fixing a problem that is quietly unraveling one of the most ecologically significant watersheds in North America.
The Battlefield: Buffalo Creek and the Lamar River Watershed
Buffalo Creek doesn't show up on most visitors' radar. It isn't the sweeping Lamar River, and it doesn't carry the prestige of Slough Creek's legendary dry-fly stretches. But its position in the watershed gives it an outsized influence on everything downstream. The closure covers the entirety of Buffalo Creek from where it enters the park in the north to its confluence with Slough Creek, which meets the main creek at the Slough Creek Campground, flowing south into the park.
Buffalo Creek will be closed to the public from the northern park boundary downstream to its confluence with Slough Creek while biologists remove invasive rainbow trout by applying the EPA-approved piscicide rotenone. The treatment zone is deliberately bounded — a surgical strike rather than a broad application — designed to eliminate the source population of non-native fish before they can continue spreading into the connected waters below.
Buffalo Creek is the source of invasive rainbow trout that have been competing and hybridizing with native Yellowstone cutthroat trout in Slough Creek, Soda Butte Creek and the Lamar River. That list of affected waters represents some of the premier wild trout habitat in the continental United States. The Lamar River system is not merely a scenic backdrop — it is a living vascular network that feeds one of the last intact high-elevation ecosystems on the continent. When the fish at the headwaters are compromised, everything downstream feels it.
Rotenone: The Science Behind the Solution
The word "piscicide" tends to make people uneasy, and understandably so. Pouring anything into a national park waterway sounds like the exact opposite of conservation. But rotenone has a long and well-documented record in native fish restoration work, and its application here is calibrated carefully.
This treatment will use rotenone, a safe, time-tested piscicide, to get rid of the invasive rainbow trout within Buffalo Creek. Rotenone is a naturally occurring compound derived from the roots of tropical legume plants, and it works by disrupting cellular respiration in fish — essentially suffocating them at the cellular level. It breaks down quickly in water through exposure to sunlight and microbial activity, and its effects on mammals are negligible at the concentrations used in fisheries work.
One of the most reasonable public concerns about any chemical treatment in a watershed is the downstream impact on drinking water. Park officials addressed that directly. The treatment will not affect the potability of water at Slough Creek Campground and will be safe to drink. The park and its biologists want the public to know that this treatment won't impact the water quality, or any fish, downstream in Slough Creek. That's a critical distinction: the treatment is a contained, upstream intervention, not a wholesale treatment of the broader drainage.
Yellowstone cutthroat trout are rarely found in Buffalo Creek, but any captured by electrofishing ahead of the treatment will be relocated outside the treatment zone. That pre-treatment electrofishing step — where biologists essentially shock the water to temporarily stun fish, then net and move any cutthroat they find — underscores the precision involved. The goal isn't to kill fish indiscriminately. It's to remove one species from a specific segment of water while preserving the native species that belongs there.
Why the Yellowstone Cutthroat Trout Matters This Much
The Yellowstone cutthroat trout isn't just another pretty fish. It occupies a singular role in the ecology of the Greater Yellowstone Ecosystem that no other species can replicate.
Yellowstone cutthroat trout are the only trout species native to the park. They are the most ecologically important fish of the Greater Yellowstone Ecosystem and are highly regarded by anglers. That combination — ecologically essential and angler-cherished — makes their decline a loss that registers across multiple dimensions simultaneously. Biologists lose a keystone species. Anglers lose a world-class fishery. And the wider ecosystem loses a food source that has structured relationships between predators and prey for thousands of years.
For thousands of years, cutthroat trout have lived in these waters, feeding bears, birds of prey, otters, and more, sustaining the intricate food web that keeps the park's wilderness thriving. Grizzly bears congregate at tributary mouths during spring spawning runs to gorge on cutthroat, building fat reserves after a long winter. Ospreys and eagles time their nesting to coincide with peak fish availability. River otters, herons, and mink all depend on the same supply. Pull the cutthroat from the equation, and the effects radiate outward in ways that take years to fully measure.
Cutthroat trout are the park's sole native trout and were the dominant fish species here prior to Euroamerican settlement. The abundance of native fish has been reduced because of impacts by introduced nonnative fish, including brook, brown, lake, and rainbow trout. These nonnative species continue to contribute to the decline in the park's native fish population by competing for food and habitat, preying on native fish, and degrading the genetic integrity of native fish through hybridization.
That hybridization threat deserves particular attention, because it's insidious in a way that outright competition is not. When rainbow trout breed with cutthroat trout, the resulting offspring are genetically compromised — they are not rainbow trout and they are not cutthroat trout. They occupy a genetic no-man's-land that, over generations, can effectively erase a species from a watershed even when fish populations remain visibly healthy. The creek can look full of trout while the cutthroat, as a genetically distinct entity, quietly disappears.
Genetically pure Yellowstone cutthroat trout populations have declined throughout their natural range in the Intermountain West due to competition with and predation by nonnative fish species, a loss of genetic integrity through hybridization and habitat degradation. The Buffalo Creek operation is as much about protecting genetic lineages as it is about maintaining population numbers.
A Conservation Fight Decades in the Making
The Lake Trout Crisis That Rewrote the Playbook
To understand what's happening at Buffalo Creek, it helps to understand what happened at Yellowstone Lake — the larger, more dramatic crisis that forced the park to fundamentally rethink how it protects its native fish.
Lake trout — native to the Great Lakes and the boreal lakes of Canada and Alaska — were first discovered in Yellowstone Lake in 1994, likely the result of an illegal introduction from nearby Lewis or Shoshone lakes, where the fish were intentionally stocked more than 100 years ago. What followed was an ecological disaster in slow motion. The lake trout invasion of Yellowstone Lake caused the native Yellowstone cutthroat trout population to crash. By 2010, 90 percent of the spawning population of native cutthroats was gone.
Lake trout are voracious predators — a mature lake trout can eat 40 native cutthroat trout over the course of a year. This not only impacts cutthroat trout, but all the animals that depend on plentiful native fish runs in the park, like grizzly bears, otters, ospreys and the like. The collapse of the cutthroat population set off a trophic cascade that reshaped predator behavior across the park. Grizzlies that had relied on spawning cutthroat as a springtime food source were pushed to find alternatives. Osprey nesting success declined. The entire food web absorbed the shock.
The park's response was the Native Fish Conservation Program, an ongoing effort that has become one of the largest and most sustained native fish restoration campaigns in American history. To maintain ecosystem function and conserve cutthroat trout, a lake trout gill netting suppression program was established in 1995, decreasing lake trout abundance and biomass. In total, 3.7 million nonnative lake trout have been removed from the ecosystem due to gillnetting since 1995.
The results, while hard-won, have been meaningful. Through these combined efforts, there has been a 79% decrease in mature lake trout in Yellowstone Lake since 2012. Yellowstone's Native Fish Conservation Program made exciting progress in their work to remove invasive lake trout from the park's ecosystem, allowing native cutthroat trout to return in greater numbers. Biologists are once again seeing Yellowstone cutthroat trout in Yellowstone Lake's streams and tributaries and have documented grizzly bears feeding on the protein-rich food source in the spring. It's a rare conservation success story — incomplete, fragile, and expensive, but real.
The History of Stocking: How the Problem Was Created
It would be easy to frame Yellowstone's invasive fish problem as something that happened to the park — a product of outside forces beyond anyone's control. The more uncomfortable truth is that humans created most of it deliberately, with the best of intentions, over the course of a century.
Many native and non-native fish were actually stocked in park waters throughout the late 1800s to mid-1900s for sport fishing purposes, which has had deleterious consequences in other regions and streams outside of Yellowstone Lake. Rainbow trout, brook trout, brown trout, and lake trout have taken hold in many lakes, rivers, and streams. The managers of the 19th and early 20th centuries were not villains — they were product of an era that viewed fish stocking as uniformly beneficial, a way to improve recreational opportunities in wild places. Nobody was thinking about hybridization dynamics or trophic cascades. They were thinking about giving anglers something to catch.
That legacy of stocking is what now drives the need for interventions like the Buffalo Creek rotenone treatment. The rainbow trout in that creek didn't swim there on their own. They are the descendants of fish put there by human hands, and removing them requires human hands as well. The 2010 Native Fish Conservation Plan formalized the park's commitment to this work, as it represents the current management direction for native fish conservation in the park.
The Broader Multi-Agency Effort
What's happening at Buffalo Creek isn't a standalone NPS operation — it's part of a coordinated, multi-agency initiative that extends well beyond the park's boundaries. The drainage that feeds Buffalo Creek originates on land managed by the Custer Gallatin National Forest, which means the invasive fish problem flows into the park from federal forest land to the north.
The Custer Gallatin National Forest along with Montana Fish, Wildlife and Parks proposes to remove nonnative rainbow trout and hybridized cutthroat trout from the Buffalo Creek drainage, just north of Yellowstone in the Absaroka Beartooth Wilderness of Custer Gallatin National Forest. After removal, the USFS plans to reintroduce native Yellowstone cutthroat trout to the drainage. The work happening inside the park's boundaries on August 31 is therefore one component of a much larger, landscape-scale restoration effort that spans agency jurisdictions and wilderness designations.
The Custer Gallatin National Forest and Montana Fish, Wildlife and Parks proposal supports Yellowstone's 2010 conservation plan. This kind of interagency alignment is genuinely difficult to achieve and represents years of coordination, environmental assessment, and public comment. Tackling a fish invasion that doesn't respect jurisdictional lines requires exactly this kind of cooperation — and it's not a given in the American West, where land management agencies sometimes have competing priorities and different regulatory frameworks.
What It Means for Anglers
For the fly fisherman planning a trip to the northern reaches of Yellowstone in late August, the closure of Buffalo Creek itself is a minor inconvenience at most. The creek is not a primary destination fishery — it's a tributary, a feeder stream, not the kind of tailwater that fills reservation books months in advance. Slough Creek, the Lamar River, and Soda Butte Creek all remain open.
But the long-term implications for anglers are enormous. Removal of the rainbow trout source population in Buffalo Creek is key to ensuring the long-term persistence of Yellowstone cutthroat trout in the Lamar River watershed. The Lamar River watershed is where some of the most sought-after wild cutthroat fishing in North America happens. Anglers who wade those waters chasing cutthroat on dry flies are fishing for a species that is simultaneously a sporting trophy and a conservation success story — or at least a work in progress toward becoming one.
The park's fishing regulations reflect just how seriously NPS takes native fish conservation as part of the angling experience. Angling is one way that nonnative fish can be selectively removed from an area without damaging the native fishery. Within the Native Trout Conservation Area, anglers are strongly encouraged, and in some cases required, to harvest nonnative fishes. This harvest will help to conserve the native fish and the natural ecosystems they support. In other words, the fisherman who keeps a rainbow trout from certain park waters isn't just having dinner — they're participating in an active conservation program.
That framing matters. It transforms the angler from a figure of potential environmental harm into a conservation partner. In Yellowstone's northern range, the guy wading the river with a five-weight rod and a net is sometimes the most effective tool park managers have for controlling invasive species in certain stretches of water. Rotenone handles the headwater creeks; anglers help manage the main rivers.
The Larger Stakes: What Happens If the Cutthroat Disappears
Strip away the fishing angle entirely, and the ecological case for protecting the Yellowstone cutthroat trout is still overwhelming. In a press release, the park said Yellowstone cutthroat are the most ecologically important fish of the Greater Yellowstone Ecosystem. That's not a marketing claim — it reflects decades of research into how this single species anchors the food web of one of the most biologically diverse temperate ecosystems on Earth.
When cutthroat populations crashed in Yellowstone Lake following the lake trout invasion, the effects were felt far beyond the water's edge. The decline of cutthroat trout in Yellowstone Lake caused a trophic cascade that affected terrestrial and aquatic food webs throughout the Greater Yellowstone Ecosystem. Grizzly bears altered their movement patterns. Osprey productivity declined. Riparian vegetation changed as bears shifted their foraging from stream corridors to upland areas. The removal of one fish species from one lake rippled through the entire landscape.
The threat from rainbow trout in Buffalo Creek works through different mechanisms than the lake trout in Yellowstone Lake — hybridization rather than predation is the primary driver — but the end result is similar: the gradual erasure of a native fish that has shaped the ecology of this watershed for millennia. The hybridized offspring that result from rainbow-cutthroat interbreeding can't perform the same ecological role their purebred cutthroat ancestors did. They exist in the water, but the relationships that the native fish had built with every other species over thousands of years — the seasonal timing of spawning runs, the nutritional profile that predators depend on — those get degraded along with the genetics.
What Comes Next
When Buffalo Creek reopens on September 7, it won't be a finished story. Rotenone treatments are effective, but invasive fish management in connected watersheds is never a one-and-done proposition. The creek's northern reach connects to forest service land where the source population of invasive rainbows still exists, which is precisely why the coordinated multi-agency effort to address the entire drainage — not just the section inside the park — is so critical.
Under the current management direction, the park would remove nonnative and hybridized trout from Buffalo Creek within the park in order to reintroduce native Yellowstone cutthroat trout. The reintroduction phase — the part where biologists actually stock genetically pure cutthroat into the treated water — is what transforms a removal operation into a true restoration. Empty water gets refilled with the species that belongs there, and then the watching and waiting begins, measured in years and decades rather than weeks.
The broader Native Fish Conservation Program carries that same long arc. Guided by science and a multi-faceted strategy, the Native Fish Conservation Program continues to confront this crisis, working to restore balance to the park's dynamic ecosystem. This relentless effort is paying off. But "paying off" in conservation biology means something different than it does in a quarterly earnings report. It means that species which were trending toward functional extinction in certain watersheds are now holding steady or slowly recovering. It means grizzly bears can be documented feeding on spawning cutthroat again in places where that behavior had disappeared for years. Progress is measured in cautious optimism rather than victory laps.
For the man who drives west every summer with a truck bed full of fly gear and a week of vacation to spend somewhere worth spending it, Buffalo Creek's temporary closure is a small thing. For the fish that have lived in the Lamar River watershed since before any human set eyes on it, the work happening between August 31 and September 7 is anything but small. It is, creek by creek and season by season, the painstaking effort to give the native West back to itself.
