In the shallows of south Lake Tahoe, diver Brandon Berry is slurping up clouds of algae with an underwater vacuum cleaner. Snorkeling above, I can hear his Darth Vader breaths better than I can see him—both researcher and lake bed are shrouded in a green muzz of metaphyton. The filamentous, cotton-candy-like algae is a persistent affliction here, where it intermittently grows, dies, washes up on the shoreline, and rots in unsightly, smelly piles.
A long string runs up out of the gloom from Berry’s contraption to the Tahoe Environmental Research Center (TERC) boat. There, chemist Michael Welsh is stationed by the on/off switch to wait for tugs on the line, as if he’s patiently fishing for research divers. It’s July, and the weather is balmy—but the researchers will be out here through the Sierra winter, huddling for warmth and wind protection in the boat’s tiny cabin and diving in heavy-duty dry suits. All to figure out how and why the famously pellucid waters of Lake Tahoe are glowing with the eerie emerald of algal overgrowth.
On the lake bed, the pale shells of one suspect shimmer through the haze: Asian clams (Corbicula fluminea), first discovered on Tahoe’s south shore in 2002 and now legion. The algae aren’t invasive, but the clams are—and the two seem to be in cahoots. As clam infestations have crept up the east side of the lake, the algae has followed.
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Aquatic invasive species (AIS) have been making trouble for Lake Tahoe’s ecosystem ever since people started sticking them in there in the mid-1800s. Invasives crowd out native plants, starve out or prey upon native animals, and kick off disastrous ecological cascades. Increasingly, limnologists are finding alliances like that of clam and algae—in which aquatic invasive species create conditions that help other undesirables spread. They’re aided by a third accomplice: climate change. Warmer waters are worse for native species, and better for invasives and potentially harmful algae. At Lake Tahoe, native fish stocks have declined, toxic algae alerts have closed down beaches, and the celebrated waters are about 30 percent murkier than they were 50 years ago. The lake’s ecosystems, along with its multibillion-dollar tourism industry, rely on clear, clean, cool water. Tahoe—jewel of the Sierra, sacred space of the Washoe Tribe, and destination for nearly six million vacationers each year (including about a million from the Bay Area alone)—is at risk.
Now, what was already one of the best-funded lakes in the West is getting an extra $17 million to fight its aquatic invaders, from the 2021 Bipartisan Infrastructure Law (BIL), via the U.S. Fish and Wildlife Service. It’s funding that Martha Williams, the agency’s director, calls a “transformational investment.” Congress also recently reauthorized the Lake Tahoe Restoration Act, allocating about $300 million to environmental projects through 2034, including $7 million for invasives control in 2025. The lake has garnered the federal money in part because “Lake Tahoe has been the example all along for how you combat aquatic invasive species, and had these partnerships and projects ready to go,” Williams tells me, adding that when the funds arrived, “everybody could start to put them on the ground and demonstrate why this investment really matters: It’s water clarity, it’s tourism, it’s cultural for the Washoe Tribe.”
The money is paying for an ambitious strategy that leverages a slew of anti-invasive weapons—but there’s still a lot we don’t know about our floating foes. That’s what is driving the TERC researchers to keep getting in the water, and puzzling out why Asian clams lurk around the algal blooms like so many tiny henchmen. A human has 86 billion neurons; an Asian clam likely has a few thousand. And yet, so far, the clams are a step ahead of us.
This is the grand Tahoe experiment: if we throw the book at aquatic invasives, can we, as the bumper stickers say, Keep Tahoe Blue?
The lake in July is a color spectrum of invasion. The water is deep blue and crystal clear at the TERC boat’s first stop, Incline Beach, on the lake’s northeastern edge; schools of silvery minnows dart above a smooth and clamless lake bed. Three miles to the south, at Sand Harbor, it’s a little greener; a few algae puffballs ripple below us, surrounded by a sprinkling of clams. It’s still clear enough to see riffles of sandy lake bed, and the glint of an old Coors Light. But when we pull up at Stateline: pea soup. “I think we’re in the right spot,” says Berry dryly: algae galore, constellations of clamshells.
We hop in. After some delicate vacuuming, Berry scoops up a dozen Asian clams and swims them up to me. The tiny invaders clink in my palm like stolen jewels. I bring them aboard to photograph as we zoom around the lake’s rim to the next survey site.
Each nickel-length clam in my hand can release thousands of microscopic larvae daily when spawning, creating infestations that outcompete native invertebrates for food. That voracious collective appetite explains why algae and clam are thick as thieves: by filtering microscopic crumbs of organic material out of the water column and excreting nutrients onto the sand, the clams create a fertilizer layer that drives algae populations far above their normal levels.
Suddenly, it occurs to me that I’m holding a dirty bomb. Sprinkled into the wrong waters, these dozen clams could be the vanguard of a new invasion, making me personally responsible for the slime-ification of some pristine inlet. Panicked, I take off one sock, stuff them into it, and tie a knot at the top for good measure. Becoming an AIS vector is alarmingly easy.
A moment later, Berry has the same thought and shouts urgently from the till: “Where did you put those clams?”
Every one of Lake Tahoe’s invasives got into the water with human help. Back in the mid-19th century, Lake Tahoe had a relatively short list of tenants: eight fish species, six of zooplankton, 12 benthic invertebrates, and a handful of plants. The lake’s top predator, Lahontan cutthroat trout, cruised the open water in search of zooplankton and smaller fish; redside shiner and dace could escape their jaws to spawn in nearshore waters.
Mackinaw trout were deliberately introduced in 1888, for anglers, and they eventually hunted the Lahontan cutthroats to near-extinction. In the 1940s, kokanee salmon were accidentally or illegally introduced by a Tahoe City hatchery; once they started spawning, the state started annually stocking the fish, which became food for the mackinaws. In the 1960s, agencies introduced mysid shrimp—as food for the kokanee salmon. This backfired: the mysids could escape the salmon in deeper waters, and they gobbled up the native invertebrates that the salmon ate. Finally, resource managers stopped messing around with Tahoe’s food web. But by then, the lake was irrevocably changed, and murkier.
Since then, most invaders have been stowaways—a strand of aquatic weed snagged in a propeller, larvae suspended in a water droplet. That’s how, in the 1980s, the bad-news Eurasian watermilfoil (Myriophyllum spicatum) and curly-leaf pondweed snuck into the lake. Then, in the 2000s, our fertile friend, the Asian clam. Nowadays, about 30 AIS have established themselves. The lake’s role as a human climate refuge is exacerbating the invasion risks. On a 100-degree day in Sacramento, twice as many cars pour into the basin as on a 65-degree day.
Timeline by Kelly Murphy
The same characteristics that make AIS easy to transmit also make them very, very difficult to remove. So an ounce of prevention is worth a pound of cure, and $5 million of the BIL funding is expanding Lake Tahoe’s watercraft inspection program, already one of the most robust in the country. (Tahoe is also a kind of anti-AIS elder sibling, handing down old equipment to other lakes to help them establish their own inspection programs.) As for the species that are here, “you’re never going to get them out of the lake,” Berry says. “Finding the best way to live with and adapt to the situation is the best course forward, as of now.” Then he shrugs. “People are smart, though. Maybe they’ll figure something out.”
Here’s some of what smart people tackling AIS at Lake Tahoe have tried: bubble curtains, benthic electrode arrays, electrofishing, skimmer robots, UV boats, weed-harvesting boats, herbicides, hydroacoustic scanning, hydrogen peroxide, drone surveys, diver-assisted suction harvesting, rubber bottom barriers, and an inordinate amount of hand-weeding.
In July, I visit Taylor and Tallac creeks, where the lake’s most ambitious AIS-removal project is underway. The largest functioning wetland in the basin, the Taylor-Tallac ecosystem is one of the few to escape being destroyed by people. Its marshes host every local native fish species and many of the region’s native birds, mammals, and plants—including the endangered Tahoe yellow cress (Rorippa subumbellata, “TYC” to its friends), a petite perennial in the mustard family that grows only on the sandy shores of Lake Tahoe, doesn’t much like humans’ beachfront developments, and has bounced back from near-extinction in California and Nevada in the past two decades. “This whole area is prime TYC habitat,” says Shannon Friedman, Tahoe Regional Planning Agency environmental improvement program manager. “It’s like the mother lode.”
We find some TYC and feel its fuzzy, fleshy leaves. As we walk, we accidentally flush out a variety of residents: a skinny garter snake, a fluffy sandpiper chick hunkered low in the grass. The black heads of Canada geese bob away above the reeds. Unfortunately, the marshes currently host every invasive species in the lake as well. “We used to say, except for Asian clams,” says Sarah Muskopf, an aquatic biologist with the U.S. Forest Service. “But now Asian clams are there too. So it’s kind of where all things collide.”
Particularly troublesome is the Eurasian watermilfoil. It grows so quickly into a dense, submerged thicket that it steals all the sunlight and crowds out native grasses and reeds. So far, it has done this in 48 states. Like an alien terraformer, it remakes the ecosystem: the masses of feathery stems slow the creek’s water down and cloud it with organic material. Native fishes, like the federally threatened Lahontan cutthroat trout, lose their spawning grounds. Invasive fishes that can thrive in the hazier, warmer habitat move in: bass, bluegill, crappie, catfish, and abandoned goldfish grown huge.
Any mere wisp of watermilfoil can spread into a new colony. It’s a kind of botanical grit that’s hard not to admire. “That’s what these invasive species are—they’re impressive,” Friedman says. Impressive, and foul. “It just gets disgusting out there, because these weeds grow, die, grow, die,” says Muskopf. “It just becomes really mucky, gross, dirty water.”
The best way people have found to fight watermilfoil here is manually, with the installation of bottom barriers: plastic tarps that lie on the creek bed like a blanket and starve the plants of sunlight, like underwater sheet-mulching. At Taylor and Tallac creeks, divers installed 17 acres of bottom barriers in 2021, the largest installation attempted in the lake or possibly anywhere. Some of the plastic barriers were so huge that they had to be lowered via helicopter to divers waiting below. The team was led by Monique Fortner, a scientist and dive safety officer from the consulting firm Marine Taxonomic Services, who tells me laying the mats was “like putting a puzzle together.” The team worked in phases: one pass to lay out the plastic tarps, one pass to put down frames around them to keep them in place, one pass to place rebar staples, and a final pass to hammer them down. They can do 25 to 30 barriers a day “when we’re really rocking and rolling,” she tells me. They dealt with a slew of only-in-Tahoe problems—like bears ripping up the plastic with their claws—while waiting three years to find out if the experiment worked. Finally, in July, the first of the mats are brought up, giving an early glimpse of what’s underneath. “Even though a lot of us have more gray hair now, I can’t even imagine the benefit that’s going to happen when we get those barriers out,” says Muskopf.
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When I visit Taylor Creek, the team is performing the installation steps in reverse. Divers move from barrier to barrier, filling their aluminum boat with debris: slimy rolls of tarp, piles of rusty rebar. The divers come in and out of the creek, shining like seals in their dark wetsuits, incongruous with the brightly colored beachside tents and kids in swimsuits nearby. A golden retriever leaps into the creek to get in on the fun, frolicking among the divers and pawing at their scuba tanks until its owner calls it off.
At the surface, the divers deliver welcome news: it’s working! Underneath the bottom barriers is bare, weed-free sand. In October, the helicopter returns, hovering over the creek to drag more of the heavy tarp out of the water before winter snows and freezes make removal impossible. The clean slate underneath will give native plants a chance to reestablish and keep the wetland healthy. The next phase is to restore the wetland’s historical flow pattern, ridding it of the warm stagnant swales that invasives love. But the restorers must also stay vigilant for the next wave of wispy terraformers. It’s only a matter of time before the creek is reinfected. “That’s just the story with invasive species, right?” says Friedman. “You do a big control effort to get a lot of it, but then it always has to be monitored and managed.”
Taylor Creek will have to be monitored more than most, because it’s right next door to an infamous invasive-weed breeding ground. If you imagine an AIS detective’s evidence board, all the red strings lead to the Tahoe Keys. When I ask people who work on invasives about the Keys, they don’t hold back. I hear it described as “crazy,” “a threat to every area,” “the prime hub of all things disgusting.”
Driving through the Keys, you could be in any affluent suburban neighborhood. Then you start to notice mast tips and white pleasure boats gleaming from behind the hedges. Once the largest wetland in the Sierra Nevada, the Keys were dredged and shaped into one of Lake Tahoe’s most expensive waterfront neighborhoods, a Californian Venice of sorts. The streets branch into the carved-out canals, ensuring that each home has street parking and boat parking. It’s a business in the front, party in the back configuration.
The Tahoe Keys endeavor has been cursed from the beginning, ecologically. When it was first built, wind off the lake constantly pelted residents with sediment and pebbles. The lagoons were shallow, mucky, and polluted, so they were dredged deeper. Ultimately, the development turned the wetland’s dynamic natural circulation into a hive of hot little backwaters—the ideal nursery for invasive plants and fish. “It was an inadvertent perfect storm,” says Andy Kopania, a part-time Keys resident and full-time water quality consultant who has been helping the Tahoe Keys Property Owners Association (TKPOA) fight the good fight against invasive weeds.
Now, those weeds cover about 90 percent of the lagoons—150 acres of infestation—and native species are all but exterminated. Instead, dinner-plate-size invasive bullfrogs vibrate in the water like lost cell phones and eat everything within tongue-shot range. Eurasian watermilfoil and curly-leaf pondweed multiply, die, and decompose, infusing the water with nutrients that fuel algal blooms, including slicks of toxic algae that can induce rashes and vomiting. From 2017 to 2023, Keys water samples contained enough toxins to prompt harmful algal bloom (HAB) advisories on 36 separate days. (Such data only hints at how often HABs have hampered people’s water activities, given that water isn’t generally sampled daily during a bloom.) The weeds’ cycle of growth and decay has fundamentally altered the chemistry of these lagoons, and the result is unrecognizable as blue Lake Tahoe water. “It has a color to it kind of like a good cup of coffee,” muses Kopania.
To witness this brew firsthand, my editor and I get out early to slip paddleboards into the Keys before the wind kicks up. At the water’s edge, my feet sink into two slimes: an underwater snarl of watermilfoil and a deeper layer of feather-soft, decomposing sludge that is the heart of the Keys’ problem. Until recently, multiple sources, like urban runoff and lawn fertilizer, were suspected of being responsible for the Keys’ nutrient-polluted water. They probably don’t help, but new sampling has proven that over 95 percent of nutrients in the Keys are in the organic sediment—the “dead weed ooze,” as Kopania calls it—at the bottom of the lagoons, where weeds have been growing and dying for decades. In a typical example of AIS efficiency, the weeds are creating their own perfect substrate.
As we paddle, fuzzy fingers of watermilfoil tangle in our keels and knot with pondweed into matted rafts. Above is the manicured decadence of the Keys—wood-paneled yachts and Jet Skis and giant inflatable beach toys float behind glass-backed mansions and the odd Trump flag. Each boat we see is berthed in its own soft bed of weeds. Serendipitously, we come across one of the weed harvester boats that have been doing their unglamorous work here since the 1970s. Passing up and down the lagoons, it chews up massive globs of plant material, pulling them up on a conveyor belt and dropping them into a sopping pile.
It’s not enough. The TKPOA has tried every counter-invasives weapon it can, and several are being tried here for the first time—bubble curtains to keep bits of weeds from drifting into the lake proper, barges harnessed with massive light arrays that blast plant-killing UV radiation, a Roomba-like robot that skims weed fragments off the surface. Still the tangles keep worsening.
Are any of the new strategies promising, at least? “That’s sort of the $60,000 question,” says Kopania (the TKPOA has actually spent more than $7 million to date). The owners’ association has been relatively successful in raising money—according to Kopania, most residents understand the severity of the problem, although support for expensive programs ebbs and flows. The sheer acreage is a bigger obstacle. The Tahoe Keys are nearly 10 times as large as the Taylor-Tallac project area, and full of nooks and crannies. The weed cutter and the UV boat can slay weeds down the middle of the channels, but they miss the infestations under docks, under boats, tucked into corners. The plants just regrow.
It’s a fractal of the larger issue: as long as watermilfoil and pondweed survive in the cranny of the Keys itself, weeds will always reinfect other areas of the lake, carried on a boat’s prop or a paddleboard’s fin, or simply drifting out into the currents. And as long as that’s the case, restoration sites around the lake will have to send divers back out, over and over again, to clean up.
Kopania hopes the new focus on the lagoon-bottom nutrient layer will be a game-changer. “In 20 or 25 years, it’ll probably look dramatically different,” he says. But then again, he points out, five years ago people thought the UV boats would be the game-changer.
Presently, the invasives have the upper hand. For Tahoe ecologists, the only option is to keep native species in the ring, hanging in there for one more round—because AIS aren’t throwing in the towel on their Lake Tahoe overhaul anytime soon. “I don’t know if it’ll be forever, but it’s probably going to be for a really long time,” Muskopf tells me. “I will be retired well before surveillance monitoring stops. You probably will be too.”
What we call long-term monitoring, of course, is just a blink in the Washoe Tribe’s millennia-long relationship with Lake Tahoe. Until miners and settlers drove them out in the mid-1800s, Washoe people summered in large gatherings on the lakeshore, building shelters out of willows, holding competitive games, and fishing for Lahontan cutthroats. Nowadays the tribe owns less than 30 acres in the basin and manages a patch of shoreline on the lake’s east side called Skunk Harbor, or Deh-iw-dee-ish-to-gahm-mum in Washo, which means “trees looking in the water.” There, Helen Fillmore, a Washoe tribal member, remembers hauling a long hose and a bicycle pump down the switchbacks through the pines to the beach with her brother, when they were kids, to try making some DIY scuba equipment (it didn’t work). Last year, the Fillmores got back in the water at Skunk Harbor—with real scuba gear—as a part of the first cohort of a Washoe Tribe volunteer dive team that is meant to join the anti-invasives strike force. “We’ve had so much fun there,” says Fillmore, who is now a TERC chemist. “And to be able to go through that training there—it feels full circle.”
The Washoe Tribe has long sought more involvement in environmental management of the basin, and the Lahontan cutthroat trout—extirpated from 95 percent of its native habitat in California—is at the center of its restoration goals, says Rhiana Jones, the tribe’s environmental program director. “As we lose access to cultural resources,” she says, “we lose part of who we are.” Now a multi-agency and tribal cooperative effort is helping the trout stage a comeback. Over the past three years, Fish and Wildlife has spent nearly $1.5 million in BIL funding to support the tribe and its environmental priorities. Some money created the volunteer dive team; another portion is helping restore Meeks Meadow, a former Lahontan cutthroat trout spawning ground on the west side of the lake.
The comeback hinges on a reintroduction effort, via hatchery. Fingerling trout are raised in Nevada, then introduced into Tahoe when they’re large enough to stand a chance. The program is still new, and Lahontan cutthroat trout take six to seven years to reach sexual maturity. So it will take a while to see if and where the trout spawn, and longer still to get the population large enough to be self-sufficient. In the spring, 10 fish were spotted exhibiting spawning behavior in a creek in Incline Village—a small but hopeful sign. “We hope to have them spawn in Meeks Creek once again,” Jones says. “If we can’t get fish to spawn, the goal hasn’t been met.”
Usually, the trout are unceremoniously piped into the lake a few thousand at a time—but in July, I join a more intimate stocking at Meeks Beach to celebrate the tribe’s connection with the fish.
First, the stars arrive: 30 young Lahontan cutthroat trout in a glass box. They shimmer pink-green-gold, speckled with dark polka dots. Tribal members and other beachgoers gather around to admire, take photos, and make jokes (“They eat anything, so even you could catch one,” says one old fisherman to another).
Washoe Tribe Chairman Serrell Smokey speaks. “This is what our ancestors fed off of for thousands of years. They’ve been overtaken by invasive species,” he says. “We’re doing our part to keep the lands and the waters as they always have been so that they can continue to keep us alive.” He describes old photographs of tribal elders holding five-foot-long trout. These foot-long youngsters have a long way to go. “Hopefully, they grow up to be big.”
Tribal youth from the local Washoe Culture Camp line up as Fish and Wildlife representatives scoop fish into blue buckets, one per kid. There are plenty of trout to go around, and soon a long procession of children is marching down to the lake to deliver the fish into its ancestral waters.
At the waterline, one young boy stares down at his bucket, into his fish’s big golden eye. “What did you name him?” someone asks.
“Bob,” says the kid, solemnly.
He dips the bucket down into the lake. For a moment, the trout seems hesitant to leave. Then it rushes out, darting between other kids’ legs in a silver flash, disappearing along the shoreline. The odds are stacked against this little fish. He’ll face warmer waters, weedy creeks, and the big mouths of hungry mackinaw. But he’s out of a concrete box and back in his natural habitat, which still has pockets where a young trout can thrive—if he can find them.
The boy watches him go. “There goes Bob.”
