Good news, just in time for Memorial Day Weekend: The clarity of the famed, cobalt-blue waters of Lake Tahoe improved dramatically last year, with visibility increasing 10 feet from the year before, a study released Thursday by scientists at UC Davis found.

The jump is the largest annual improvement in 50 years, since measurements at the iconic Sierra Nevada lake began in 1968.

On average in 2018, the study found, a 10-inch white disk lowered from a research boat was visible 70.9 feet below the water’s surface. A year before, the disk could be seen only up to 60.4 feet, the lowest visibility level ever recorded.

The reason for the huge loss of clarity in 2017, scientists said Thursday, was that heavy rains in the winter of 2016-17 washed massive amounts of sand and mud that had built up during California’s five-year drought into the lake. In fact, more sediment washed into the lake in 2017 than the previous five years combined. The big drop-off in the lake’s clarity alarmed environmental groups, tourism leaders and many Tahoe lovers.

“A lot of people last year were looking at the decline and saying, ‘It’s not working, Tahoe is not getting better,’ ” said Geoffrey Schladow, a professor of engineering at UC Davis and director of the Tahoe Environmental Research Center. “Many of us were saying it was a really extreme year in 2017. It was a massive end to a massive drought. There was a lot of new material coming into the lake and lots of new erosion. Now the lake has returned to being even better than it was. It’s good news.”

In 2018, after a relatively mild winter, not as much sediment washed in, returning the lake to a more normal pattern, he said.

Lake Tahoe’s recovery is measured in decades, not years. Overall, the lake still has a long way to go to recover the level of clarity it had half a century ago. In 1968, Tahoe’s visibility was 102.4 feet.

After steady development that began in the 1920s and accelerated in the 1950s, 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.

Over the past 20 years, the state, federal and local governments have spent hundreds of millions of dollars restoring wetlands, tightening building rules and making other changes to try and stop the 1,645-foot deep lake — America’s second-deepest, behind Crater Lake — from becoming a muddy green mess of algae and silt.

That work, highlighted around the United States in the “Keep Tahoe Blue” bumper stickers, has shown slow but steady progress.

The five-year average in lake visibility — widely considered an indicator of the Lake Tahoe basin’s environmental health — is now 70.3 feet, an increase of almost a foot from the previous five-year average.

This winter has been wet, with the Sierra Nevada snowpack currently at 148 percent of its historic average. Does that mean the lake’s clarity will drop in 2019?

“It possibly will go down,” Schladow said. “But it’s the long term-trend that matters. A lot of the investments at Tahoe have been made to stopping fine particles from urban, built-up areas from entering the lake.”

Thursday’s news drew cheers from environmentalists.

“We are thrilled,” said Darcie Goodman Collins, CEO of the League to Save Lake Tahoe. “These results encourage us to continue restoring critical habitat and improving our urban areas to keep pollution from entering our lake.”

Lake Tahoe is a key tourist attraction, with roughly 3 million people visiting each year. It’s also a natural wonder. If the Empire State Building were submerged in Lake Tahoe, the top of its spire would still be below 200 feet of water.

Despite the good news Thursday about the lake’s improving clarity, Tahoe still faces significant challenges. One of the most troubling is that the lake’s waters are steadily warming as the Earth’s climate continues to heat up.

In 2017, the average surface temperature at Lake Tahoe was 53.3 degrees Fahrenheit, up from 50.3 in 1968.

Scientists say the warming water probably will result in more algae growth, silt from drying soils, invasive species and fire risk in the forests around the alpine landmark.

“Research shows Lake Tahoe and other inland water bodies are warming faster than the oceans and atmosphere,” said Joanne Marchetta, executive director of the Tahoe Regional Planning Agency, a government agency established in the Nixon administration to regulate land use around the lake.

“So it’s imperative we continue to invest in the lake’s restoration to combat new and emerging threats.”

One promising way to offset the future visibility loss from climate change might be by removing a tiny species of shrimp from the lake.

Generations ago, fisheries managers introduced an invasive species of shrimp, called Mysis, into Lake Tahoe and other Western lakes as a way to fatten up trout and other fish for recreational fishermen. But those shrimp ravenously ate a beneficial, native type of zooplankton, called Daphnia.

The plankton eat algae. Fewer plankton mean more algae. Researchers from UC Davis found that from 2011 to 2016, when the population of the half-inch shrimp declined in Emerald Bay for reasons they still don’t fully understand, clarity increased dramatically in that part of Lake Tahoe as the plankton numbers rebounded.

Now, UC-Davis scientists are using sensitive sonar equipment to find where the shrimp are concentrated in the lake. They designed a special trawl net and have been removing them by essentially fishing for them. The studies are continuing. But so far, they suggest that removing from 50 to 70 percent of the shrimp allows the helpful plankton to return. The experiment holds “tremendous potential” for restoring lake clarity in future years, the researchers wrote in a report last year.

“Climate change can get depressing at times if you don’t have any hopeful leads,” said Schladow. “For us, this is one.”