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Lake Tahoe’s iconic blue waters were the third murkiest on record last year and the worst they’ve been in several years, according to data from scientists who have studied the lake for decades.
Clarity of the alpine lake — measured by dropping a white disk into the water and noting when it disappears from sight — is a signal of its overall health. Tiny particles are major culprits of reduced clarity, including the sediment and other pollutants that wash into the lake from runoff and air pollution and the plankton that grow in its waters.
Researchers with UC Davis’ Tahoe Environmental Research Center reported today that the average murkiness in 2024 was exceeded only in 2021, when fires blanketed the lake in smoke and ash, and in 2017, when the lake was clouded by sediment-laden runoff during a near-record wet year.
The report says that clarity levels are “highly variable and generally not improving,” and recommends that “future research should focus on examining the nature of the particles that affect water clarity.”
In 2024, the white measurement disk disappeared 62.3 feet from the surface — about six feet shallower than the previous year, and only 1.9 feet deeper than the worst record set in 2017.
Last year’s average clarity was about 40 feet worse than in the late 1960s, when scientists began tracking the long decline.
Lake Tahoe, surrounded by snowy slopes in the winter and forested trails in the summer, draws 15 million visitors every year to enjoy boating, skiing and hiking — three times more than Yosemite National Park.
It’s the clearest large lake in the world, and one of the deepest. Yet despite billions in state and federal dollars spent to improve Lake Tahoe’s health, its famed clarity dropped steeply through the 1990s and, though it leveled off, has not improved. The 2024 levels fall about 16 feet short of the state’s clarity goals for 2031, and will need to improve by 35 feet to return to historical levels by 2081, according to the state’s Lahontan Regional Water Quality Control Board.
“The water clarity is not getting noticeably worse, but it’s also not getting better. So the big question for us now is why.”Stephanie Hampton, UC Davis
“I see something that we should celebrate, which is the success of management and restoration in slowing or potentially halting the decline,” Stephanie Hampton, director of the Tahoe Environmental Research Center and a professor of environmental science and policy at UC Davis, said.
Still, she added, “the water clarity is not getting noticeably worse, but it’s also not getting better. So the big question for us now is why.”
The lake is clearer in winter than in summer. But last year, both winter and summer measurements were the third worst seasonal measurements on record.
The winter murkiness came a year after the lake had shown an unusually dramatic wintertime improvement — nearly 92 feet of clarity in winter of 2023, compared with about 72 feet in winter of 2022 — because of deep mixing that brought clear water to the surface.
The murkiness, however, reappeared that summer. And in 2024, summer measurements brought a mystery: Even after particle levels dropped, the lake didn’t get clearer, “suggesting unobserved factors or more complex dynamics affecting summer clarity,” the report said.
“That caught our eye, because it made us wonder, is there something that we’re not measuring that is affecting the clarity?” Hampton said. One possibility is that tiny plankton — smaller even than the ones the scientists are tracking — could be keeping the water murkier than expected.
“Tahoe is changing in several ways all at once,” Hampton said. As climate change continues, “the water is getting warmer, summer is getting longer, and that could boost some of that growth.”
Sudeep Chandra, director of the Global Water Center at the University of Nevada, Reno, said the continued decline of summer clarity means that things are more complicated than previously thought and more research is needed.
“Scientists continue to sound the alarm at Tahoe and elsewhere that we need support to not just measure the heartbeat or pulse of the lake by measuring clarity, but also the other vital organs,” said Chandra, who is not involved with the UC Davis clarity research. That includes the watershed and streams, and the lake’s ability to process nutrients and other materials that wash into it, and the effect of climate change on snowpack and drought.
Many environmental and community groups have criticized the Tahoe Regional Planning Agency — a California-Nevada governmental agency created in 1969 to protect Lake Tahoe — for allowing commercial development, tourism and growth.
For instance, the Tahoe Blue Event Center, which opened in Nevada last year, is a multi-purpose, 5,200-seat arena with an expansive luxury condominium development planned across the street.
The planning agency has “gotten off track of its core mission, which is the lake and the quality of the lake water, and the surrounding environment of the lake… They are listening to developers too much,” said Tobi Tyler, vice chair of the Sierra Club’s Tahoe Area Group. “It’s being loved to death, really.”
But officials with the Tahoe planning agency have said development projects don’t detract from their efforts to understand and restore the lake and its surroundings.
“The lake is still visible to 60 feet of clarity — it’s incredibly clear, and that’s something that we’ve managed to stop from further harm,” said Jeff Cowen, public information officer with the Tahoe Regional Planning Agency.
“It’s concerning to see that there are trends that are not reacting to the investments…We want to know what more we need to do.”Jeff Cowen, Tahoe Regional Planning Agency
“The clarity loss is like a locomotive where they took the steam off the engine but it’s still rolling along. And to see it stabilize is … a pretty good sign of progress.”
Cowen said roadway work has helped keep more than half a million pounds of sediment from reaching the lake. Nutrients that fuel algae growth have also been cut by thousands of pounds every year, and more than 1,400 acres of improved or restored wetlands help filter out pollutants and reduce erosion.
“It is concerning, for sure… We’re leaning on the science and the research, and it’s concerning to see that there are trends that are not reacting to the investments,” Cowen said. “We want to know what more we need to do.”
This article was originally published on CalMatters and was republished under the Creative Commons Attribution-NonCommercial-NoDerivatives license.