Lake Tahoe considers 'controversial' herbicides for the first time in fight against aquatic weeds

There are 1,500 homes in the Tahoe Keys. Many of the homes have piers stretching from their backyards into waterways that connect to Lake Tahoe. Before these homes and canals were built, this land was full of meadows and marshes that made up the largest wetland in the Sierra Nevada.

The wetlands once played a fundamental role to filter sediment and pollution from a major tributary into Lake Tahoe, the Upper Truckee River.

Now, officials have identified the Tahoe Keys as the “primary source” for an infestation of weeds that has already started to march into Lake Tahoe and threatens to spread quickly.

Last July, the Tahoe Regional Planning Agency and the Lahontan Regional Water Quality Control Board published a draft environmental impact statement for a control test to fight the aquatic weeds. The solution that’s being proposed is controversial and has never been done before. If approved, the Lahontan water board would grant an exemption to a rule that prohibits herbicides in Tahoe so they could be used in a test to measure their efficacy to destroy the weeds before the infestation gets worse. Opponents like the Sierra Club call the herbicides a "Band-Aid" solution that could have greater consequences. But it's a last resort option that even the League to Save Lake Tahoe — a staunch defender of the lake’s clarity and ecology — supports, with conditions.

“Some stakeholders believe using aquatic herbicide is the only way to confront a weed infestation of this magnitude,” reads the environmental impact statement.

I wanted to learn more, not just about the herbicides and the weeds, but how a development built on important wetlands started this big issue in the first place. That's how I wound up in the Tahoe Keys to meet representatives of the League to Save Lake Tahoe and the Tahoe Keys Property Owners Association. They said they could show me the problem, which is clearly visible beneath the water.

I parked my car on the side of a cul-de-sac and walked across the street to meet Jesse Patterson and Chris Joseph, chief strategy officer and communications manager, respectively, of the League to Save Lake Tahoe. They led me down a ramp toward the water where a pontoon boat was waiting for us. On board was Greg Hoover, who supervises the weed control operations for the Tahoe Keys Property Owners Association. This group has been working collaboratively with the Lahontan water board and the Tahoe Regional Planning Agency, among other groups, on the complex and controversial project to cut back the aquatic weeds in the Tahoe Keys and stop the infestation’s spread in Lake Tahoe.

As soon as the pontoon left shore, a mess of oozing, stringy, brown muck spit out from beneath the boat. The plants swirled at the surface of the water, like a clump of seaweed except we were hundreds of miles from the ocean. As we motored out into the canals, I could see strings of plant life circling along the water’s surface and gathering in slimy clumps everywhere.

The homeowners in the Tahoe Keys have been dealing with this mess since the 1970s at least. It’s Hoover’s job to harvest and control those weeds, which are mostly Eurasian watermilfoil, coontail and curlyleaf pondweed. His method is essentially to mow the water, he explains. Imagine a floating lawn mower on a paddle wheel that chops the top four to five feet of the plants off.

Hoover told me his staff mows the water and cuts back the weeds five days a week in the summer. (The Tahoe Keys are mostly frozen over in winter.) But it’s not very effective. Like a lawn mower, the harvesting boat leaves fragments of the plants in its wake. Those fragments float to the bottom and seed new plants, making the issue worse.

“One fragment creates one new plant,” Hoover said, explaining the process to me as the pontoon steadily motored through the canals. “That’s just the way it works.”

The weeds are out of control and have profoundly altered the ecosystem in the Tahoe Keys. The infestation has spread to 172 acres of the subdivision’s waterways.

Although the Keys are man-made, they still have qualities of a wetland with a marshy bottom and warmer water. The invasive aquatic weeds are changing the nutrient concentrations and temperatures of the water. The growth is so dense, it’s difficult for boats to navigate through the water, not to mention other health and safety risks associated with overgrown plant material in dark waters. Native fish species are also in peril.

And now, in large part because of boat traffic from the Keys, the weeds have already spread across 100 acres in Lake Tahoe. The infestation has been growing steadily in recent years and is fast catching up to the size of the problem inside the Keys. Last October, the League to Save Lake Tahoe, the Tahoe Regional Planning Agency and the Tahoe Resource Conservation District teamed up with a combination of public-private funding to dispatch scuba divers with the Marine Taxonomic Services to Lake Tahoe to pull up the invasive weeds by hand and with “suction-dredging” equipment. Patterson described it to me as if scuba divers had shop vacs.

“We’re going to see what grows back,” Patterson said in a phone interview the first week of January. The scuba diver cleanup will be expanded next summer, too. The method is expensive and time-consuming, but hopefully it’s effective.

To help prevent further spread of the infestation, the Keys installed a “bubble curtain” at the mouth of the west channel, which connects the residential lagoons to Lake Tahoe. It’s like a v-shaped underwater wall of bubbles that’s designed to dislodge fragments of the weeds from the underbellies of boats leaving the Keys. Then, seabins, or “floating trash cans” as the League describes them, collect the fragments before they land and seed new plants. A second bubble curtain is going to be installed on the east channel, the entrance between the Keys marina and the lake, this winter.

Patterson pointed out that the problem in the lake is vastly different from the problem inside the Keys. Divers can still come in and pluck weeds from Lake Tahoe, but they couldn’t do that in the Keys because it’s so overgrown and so mucky that any disturbance would lead to a blinding black swirl of mud.

That’s why officials are looking to use a last resort option in the Tahoe Keys: herbicides.

Herbicides are prohibited in Lake Tahoe, which is designated as an Outstanding National Resource Water. That means that Lake Tahoe is held to strict water quality standards, including a ban on any element that would degrade the water.

But the weed problem has gotten so bad that in 2017, the Tahoe Keys Property Owners Association submitted an exemption application to the Lahontan Regional Water Quality Control Board. The idea is to implement a test that's small in scope and will help officials see the impact herbicides might have to control the weed infestation.

The test would look at a variety of methods in different control plots. In addition to aquatic herbicides, the test would also include UV light and a method called laminar flow aeration that removes nutrients from the sediment. Patterson said the environmental study for the test cost more than $1 million. It would be a three-year test with a two-year observation period. After the test, officials expect to use the results to do another environmental review for a larger-scale project to contain the weed infestation at large throughout the Tahoe Keys.

“It’s a huge deal. We want to make sure it’s done correctly and done in a way that addresses the aquatic invasive species threat,” Patterson said. “We’re not interested in using [herbicides] in perpetuity or using them anywhere else but the Keys.”

Patterson said the league is comfortable with the test’s use of the herbicides because the concentrations are low and it would be a one-time use intended to knock the weeds back to a level that would make them manageable with other non-herbicide methods. The league does not support using the herbicides permanently.

“That’s too big a risk,” Patterson said.

A decision to approve the draft environmental impact statement and the test project was originally slated for later this winter, but the Lahontan water board delayed a decision until summer 2021. The delay bumps back the test project until spring 2022, at least.

“Number one, we want to make sure that the environmental review and that the agencies are doing their due diligence so we get the best test,” Patterson said. “We build public confidence in the process. We build the test in a way that will tell us what a long term solution will look like.”