The clarity of Lake Tahoe — the famed alpine lake on California’s border with Nevada, whose spectacular scenery draws millions of visitors a year and has spawned countless bumper stickers to “Keep Tahoe Blue” — is in the middle of a curious trend.

It isn’t really getting much better. Or much worse, despite relentless efforts to improve it. And scientists aren’t sure why.

A new study published Monday by researchers at UC Davis shows that the annual average clarity for Lake Tahoe’s azure blue waters in 2024 was 62.3 feet, measured as the depth to which a 10-inch white disk, called a Secchi disk, remains visible when lowered into the water. That’s slightly worse than the previous year’s average of 68.2 feet. But over the past 20 years, the clarity — widely considered a measure of the lake’s overall health — has moved up and down by a few feet a year but generally remained stable.

“We should celebrate the success that we’ve had that has slowed and possibly halted the declines in clarity,” said Stephanie Hampton, director of the UC Davis Tahoe Environmental Research Center. “But why isn’t it getting any better?”

There are some clear trend lines, she noted. The lake is getting more clear in the winter. And less clear in the summer. But numerous new questions remain, including how the lake’s waters, which are slowly warming as the climate warms, may be becoming more favorable to algae growth. Or what role ash and other particles from large wildfires are playing. Or whether microscopic pieces of plastic may be playing a role.

“Lake Tahoe is truly magnificent,” Harmon said. “It is jaw-dropping and beautiful. We want to make it healthier and reverse the declines.”

At 1,645 feet deep, Lake Tahoe is the second deepest lake in America, behind only Crater Lake in Oregon, which is 1,949 feet deep. Over the past half century, the lake’s clarity has become a rallying cry, not just for environmentalists, but also for the business community, which depends on tourism, the linchpin of the region’s economy.

In 1968, the circular disk the size of a dinner plate that UC Davis researchers use to measure visibility in Tahoe’s waters could be seen 102.4 feet below the surface.

After decades of decline when it fell to 85 feet by 1975, then 79 feet by 1985, federal, state and local officials launched a massive effort starting in 1997 to cut the amount of sand, dirt and other sediments flowing into the lake from roads, construction sites and other sources.

Since then, officials have rebuilt stormwater systems around the lake to capture sediment so it doesn’t flow into the water, required developers to reduce erosion, built bike lanes and expanded bus service to reduce traffic — because vehicle exhaust contains nitrogen which can boost algae in the lake — and have thinned more than 90,000 acres of forests in the Tahoe Basin to reduce fire danger.

Between 2004 and 2023, the amount of sediment going into the lake fell by 30%, a drop of about 535,000 pounds a year, said Jeff Cowan, a spokesman for the Tahoe Regional Planning Agency, a government organization established by Congress to regulate development in the Tahoe area.

Similarly, the amount of nitrogen fell 18% and phosphorus fell 20% over the same time, he said.

Now that the decline in clarity has leveled off but not improved, scientists and policymakers say they want to go back and study the tiny particles in the water more closely to better understand where they are coming from and how they are affecting the lake’s clarity.

“We want to know if we are doing the wrong things or not doing enough of the right things,” Cowan said.

The lake seems to become clearer during droughts, when less water carrying sediment and nutrients flows into it. In wet years, particularly after wildfires have burned parts of the landscape, more water flows in, and clarity drops.

For generations, people have wondered if the lake is being loved to death. In the 1860s, Mark Twain called Tahoe “the fairest picture the whole earth affords.” But loggers clear-cut many of the trees around it to provide supports for silver mines in Nevada.

Steady development that began in the 1920s accelerated in the 1950s, with casinos, Frank Sinatra, Marilyn Monroe and the Kennedys joining millions of visitors. Tahoe’s waters started getting murkier because of erosion from construction, fertilizer from golf courses, and loss of wetlands that filter pollutants and other human disruptions.

In the 1990s, former Sen. Dianne Feinstein of California and Harry Reid of Nevada led efforts to hold an annual summit at the lake and expand funding for wetlands restoration, forest thinning and other projects.

Local and state officials set a goal in 2010 to get the lake’s clarity back to 97 feet by the 2060s.

Among the key questions now are learning more about what causes the lake to periodically mix in a major way, when cold clear deep waters move to the surface; trends of different types of organisms, from shrimp to plankton; along with the temperature change. The average temperatures of Tahoe’s surface waters has risen 3 degrees since 1968, up from 50 degrees then to 53 degrees now.

“We need to dig deeper and learn more about the processes and interactions going on below and above the surface,” said Darcie Goodman Collins, CEO of Keep Tahoe Blue, an environmental group. “What role do algae and phytoplankton play in clarity? What role does wildfire smoke and ash play? What about the effects of microplastic pollution and aquatic invasive species?”

“These are questions that don’t have easy or fast answers,” she added. “But anyone who has ever experienced Tahoe knows the effort is worth it.”