The lake supports a $5 billion economy and a stunning natural eco-system. But all the beauty is facing a growing underwater threat -- plants and animals that are not supposed to be there.
There are about 20 invasive species in Tahoe, crowding out native species and changing the chemistry of the water.
One invasive weed is Eurasian watermilfoil. It creates an underwater jungle that clouds the famously clear water and gets tangled in boat propellers.
The worst infestation is in the Tahoe Keys, a private community with man-made lagoons that feed into the lake.
Harry Dotson is part of a homeowners group there.
"We remove approximately 5,000 cubic yards of weeds every year," Dotson said. "The total cost of that operation is about a quarter of a million dollars per year."
One way to control invasive species could be pesticides. But Tahoe has some of the strictest environmental rules in the nation. The only pesticide allowed is for mosquito control in stagnant ponds that surround the lake.
No poison is allowed directly in Tahoe -- at least not yet. The Lahontan Regional Water Quality Control Board now wants to permit pesticides as a last resort.
"The idea is to apply them in a manner that's very focused and then the products do break down," board spokesperson Lauri Kemper said.
Under the new rules, pesticides could be used only if non-chemical methods are not effective.
Many environmentalists don't like it.
"We're very concerned with poisoning Lake Tahoe," California Watershed Network spokesperson Laurel Ames said. "There's really no way to control liquid poison in a lake. It's going to move. There's just no way they can keep it in one specific spot."
The 12 agencies that provide drinking water from Lake Tahoe are also fighting to keep pesticides out.
"Currently there is no treatment for us to be able to get that stuff out of the water once it gets in; so what we would prefer is that it stays out to begin with," Tahoe Water Suppliers Association spokesperson Greg Reed said.
The regional water board says there are no immediate plans to use pesticides in Tahoe, but they need regulations in place just in case. They've joined other Tahoe agencies in a task force to find non-chemical solutions for invasive species.
"The goal of this project is really to make it so we don't have to use pesticides," environmental scientist Dan Sussman said.
The task force is now working on an experiment in Tahoe's Emerald Bay. They put rubber mats on the bottom of the lake to smother invasive Asian clams. It's still too early to know how well it worked.
Back at Tahoe Keys, mats on the bottom of the lagoon might also fight milfoil, but homeowners are skeptical.
"You know you are just talking millions and millions of dollars to do that," Dotson said.
Dotson thinks pesticides are the most viable answer and points out they're used in many other lakes.
But Lake Tahoe is an "outstanding national resource water" -- a legal status that gives it very special protection.
That protection will be tested Tuesday when the state water board votes on the pesticide plan. If they approve it, it would still have to get the final go ahead from the Environmental Protection Agency.
Written and produced by Jennifer Olney
