Can herbicides keep Tahoe blue?

A new chemical weed management plan has the lake’s water suppliers nervous.


On a hazy July afternoon a few miles west of the California-Nevada border, Lars Anderson paces the docks that frame Lake Tahoe’s southern shore, twirling a rake in his right hand, preparing to pull some weeds.

With a flick of his wrist, the plant physiologist flings the rake, tethered to an orange rope, 20 feet out into the lake’s opaque waters. Anderson hauls hard and the rope twangs taut, as though he’s hooked a gigantic fish. At last his catch emerges: a dripping mat of blackish vegetation, longer than Anderson is tall, its tendrils dense and clotted as hair plucked from the drain. Anderson lays it on the dock, where it shimmers like a dead deep-sea squid.

The plant, Anderson pronounces, is Eurasian milfoil, an invasive species. He picks through the snarl, finally plucking a green needle from the slimy haystack. “Native elodea,” he says, holding up a pitiful sprig. “This is probably 5,000-to-1, non-native to native.” Speedboats line the docks of the marina; cookie-cutter houses squat along the shore. “If we didn’t do anything, this whole thing would be covered in plants,” Anderson says. “You couldn’t even get a boat out.”

Welcome to the Tahoe Keys. 

Lars Anderson pulls aquatic weeds out of the Tahoe Keys Marina.
Kat Kerlin/UC Davis

Lake Tahoe is famous for many things: world-class skiing, slightly less world-class casinos, Frank Sinatra’s rowdy Rat Pack. Above all, however, it’s famous for being blue. Writing in 1863, the physicist John LeConte was astounded by its lucidity and spectral diversity; he observed “cyan-blue (greenish-blue), Prussian blue, Cobalt-blue, genuine ultramarine-blue,” and even “Marie-Louise blue,” a tribute to the color of Napoleon’s army’s uniforms.

In the Tahoe Keys, however, the water takes on a murkier cast, as though a few dollops of brown paint have mixed with the indigo. The Tahoe Keys is a manmade 750-acre housing complex that was dredged from the wetlands along Tahoe’s southern shore in the 1960s; these days, it’s described by the Tahoe Daily Tribune as an “amalgam of islands and peninsulas with no particular symmetry at all.” Invasive species readily colonized the Keys’ warm, shallow artificial channels, particularly Eurasian milfoil, which arrived in the 1980s. The weedy mats entangle propellers, crowd out native vegetation and spawn algal blooms by pumping nutrients into the water column.

“When I first moved out here, you’d see a hundred kids swimming in the Tahoe Keys canal — now, you wouldn’t let your dog swim in there,” says Steve Urie, a resident of nearby Truckee who’s lived in the area for over 40 years. “If we didn’t have the Tahoe Keys, there wouldn’t be an invasive species issue.”

Milfoil also shelters other invaders, like bluegill, largemouth bass and even gargantuan goldfish. Not only has the Keys been itself overrun, it’s become a locus of invasion for the entire lake: Scientists think that boaters accidentally introduced milfoil to nearby Emerald Bay State Park, among other sites. In recent years, California’s drought has encouraged the plant’s growth by reducing lake levels and exposing more habitat. Another hardy invader, curlyleaf pondweed, is spreading, too.

“Weeds have a tendency to get up a head of steam, and their expansion can be exponential,” says Joel Trumbo, senior environmental scientist at the California Department of Fish and Wildlife. “Sooner or later, they’ll move out of the Keys and find suitable habitat in other places around the lake and cause ecological havoc.”

Now lake managers are considering more aggressive tactics to control the invasion. The Tahoe Keys Property Owners Association, a residential community comprising 1,500 homes, already spends $400,000 annually on harvesters — floating mowers that remove the plants. But the harvesters haven’t succeeded; in fact, they may actually promote milfoil’s spread, by creating plant fragments that settle and establish elsewhere. Call it the Sisyphean approach to weed control.

That’s why Anderson and others want to try a technique alien to Lake Tahoe: herbicides. The Tahoe Keys Integrated Weed Management Plan — released in August by the Property Owners Association — mentions the usual mechanical strategies, including better harvesting, scuba divers pulling up plants, and bottom barriers, mats that block sunlight and smother weeds. But it also relies heavily on chemical control. “The problem is big enough that we need to consider all available methods, and that includes herbicides,” says Anderson, a former Department of Agriculture aquatic weed specialist who co-authored the plan as a consultant to the Association.

Though the Keys has yet to decide on a chemical, the draft management plan mentions five polysyllabic options, such as imazamox and penoxsulam. Trumbo, one of five independent experts who reviewed the plan, attests to its safety. “Herbicides are directed at plant physiological processes,” he says. “The potential herbicides they’d be using on this project run from slightly toxic to fish down to practically non-toxic — the two lowest categories. When it comes to mammals, it’s largely the same thing.”

Trumbo adds that herbicide concentrations will be low enough, and dilution rapid enough, to keep the odds of dangerous human exposure vanishingly small. And if monitoring suggests that herbicides may escape the Keys’ canals and enter the rest of the lake, giant curtains can be lowered to contain the chemicals’ flow.   

Next month, the Keys plans to submit the Weed Management Plan to the Lahontan Regional Water Quality Control Board, the regulatory body that governs Lake Tahoe. Though Lahontan had long prohibited the application of pesticides in Lake Tahoe, in 2011 it created an exemption, motivated by the arrival of invasive quagga mussels in nearby Lake Mead.

Still, the exemption is no rubber stamp: “Herbicides have never been used before, and people are going to want to make sure it’s being done carefully, and to the minimum extent necessary,” says Lauri Kemper, assistant executive officer at Lahontan. If the board OKs the Keys’ weed plan, the herbicides could start flowing in 2017 and continue through 2020.

Predictably, not everyone is delighted at the prospect of dousing Lake Tahoe with chemicals. Down by the docks, 50 feet from where Anderson harvests his weedy bounty, Madonna Dunbar, a pink-faced woman in a broad sunhat, discourses on why the herbicides represent a potential catastrophe. Dunbar serves as executive director of the Tahoe Water Suppliers Association, a group that represents 11 Lake Tahoe municipal water providers. Dunbar’s constituents purvey some of the country’s finest water: Beyond the Keys, the lake’s water is so clean that six water suppliers have a rare filtration exemption from the EPA.

Dunbar believes it would be madness to jeopardize that purity, either in reality or in reputation. After all, Rachel Carson and Theo Colborn, the scientist who helped alert society to the perils of endocrine disruptors, have taught us that categorically guaranteeing the safety of chemicals is a dangerous game. Penoxsulam, for instance, is classified as “Suggestive Evidence of Carcinogenic Potential,” which means that it may cause tumors in rodents, though there’s “not enough information to extrapolate the risk to humans.” According to one analysis, water droplets from the Keys can reach nearby drinking water intakes in less than 24 hours.

 “We have a long history of spending billions of dollars to prevent things from entering Lake Tahoe,” Dunbar says. “This would be a total shift in philosophy about what’s allowed here.”

Dunbar agrees that milfoil poses a serious problem — she just believes that chemicals should remain a last resort. “That’s what it means to be an integrated plan, and not just an aggressive herbicide plan,” she says. Among other techniques, she advocates retrofitting harvesters into rototillers that would churn up the bottom and dislodge plants by their roots, a method that the Okanagan Basin Water Board has used in British Columbia to control milfoil. (In response, Anderson points out that Okanagan used the rototillers in a swimming area, rather than in a sensitive ecosystem.)

It’s true that mechanical removal worked in Emerald Bay, the state park that was invaded by milfoil along its western shore. There, teams of divers hand-pulled a million stems over four years; in 2015, Dan Shaw, an environmental scientist at California State Parks, has not seen any milfoil in Emerald Bay. “It really has exceeded our expectations,” says Shaw, though the battle is far from won. “We have a thousand boats a day coming into Emerald Bay, and they could all be carrying (aquatic invasive species) — especially if they’re coming from other infested areas.” 

Emerald Bay’s experience, however, may not apply to the Tahoe Keys. Hand-pulling weeds is a laborious process; meanwhile, according to the weed plan, it would cost a whopping $65,000 to $100,000 per acre to install bottom barriers in the Tahoe Keys. By comparison, says Sudeep Chandra, a biologist at the University of Nevada-Reno who’s studied the lake for over a decade, herbicides are “more effective and much cheaper.”

Chandra didn’t form that opinion idly — for years he’d refused to even consider herbicides in Lake Tahoe. “But seeing the amount of invasive species movement around the lake, and how it affects near-shore water quality, has made me change my position,” he says. “The problem has gotten bad enough that we need to have multiple tools in the toolbox.”

For decades, Lake Tahoe’s advocates have been united in their approach to water quality. They’ve fought the buildup of impervious pavement that leads to polluted stormwater runoff; they’ve promoted forest health; they’ve battled the influx of sediment from rampant development. (In addition to the weed plan, the Keys is also working on a nutrient management strategy to curtail pollutants that stimulate weed growth.) In many ways, those efforts have paid off: According to UC Davis’ 2014 State of the Lake report, last year Tahoe was the clearest it had been in over a decade.

“We’re gaining a better understanding of how Lake Tahoe functions, and our efforts to keep it blue and clear are starting to work,” says Geoffrey Schladow, director of the University of California, Davis Tahoe Environmental Research Center.

At least where the Tahoe Keys is concerned, however, consensus has broken down. A strain of “us versus them” permeates the rhetoric pitting Keys residents against other lake users: the anti-herbicide Facebook group with nearly 1,500 members, the op-eds asserting that “poisoning our lake for the convenience of homeowners … is not the answer.” Herbicides seem to have widened existing divisions in a watershed where multimillion-dollar mansions and working-class communities crouch in tense juxtaposition, where year-round residents and Bay Area vacationers rub uneasy elbows.  

Water quality in Lake Tahoe is virtually a religion, and “Keep Tahoe Blue” its mantra, proclaimed on bumper stickers across the West. Everyone — blue-collar or white-, full-time or time-share — is passionate about water. Yet different factions prioritize different qualities: Some want their lake clear and weedless; others are most concerned that it remains chemical-free, now and forever.

A development as harmful as the Tahoe Keys would never survive the permitting process today, but it’s too late to reverse course now. Indeed, that’s the nature of conservation throughout the modern West, where preservation has ceded to the management of fragmented landscapes and compromised waters.

“There’s some deeply entrenched people who would like to return the Tahoe Keys to a wetland,” acknowledges John Larson, president of the Property Owners Association’s board of directors. That’s not going to happen. Still, says Larson, “In the last few years, we’ve been working hard to accept responsibility for what we are, who we are, where we are. Dealing with weeds is part of that. It’s something we have to do.” 

Ben Goldfarb is a correspondent for High Country News. 

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