SOUTH LAKE TAHOE, Calif. — Lake Tahoe’s clarity has remained relatively stagnant, said a 2020 report, but local agencies are fighting to win back the crystal clear waters.

“Lake Tahoe’s water clarity measurements, which are indicators of the health of the watershed, averaged 62.9 feet through 2020,” a joint press release from the UC Davis Tahoe Environmental Research Center and the Tahoe Regional Planning Agency stated.

Lake Tahoe’s clarity peaked in February 2020 when it was deeper than 80 feet. It was at its lowest in mid-May when it measured at slightly more than 50 feet. These readings were within the average range of the last decade. Average clarity in 2020 was just slightly better than the previous year’s average of 62.7 feet.

Clarity has been measured by UC Davis researchers since the 1960s as the depth to which a 10‐inch white disk, called a Secchi disk, remains visible when lowered through the water. Because lake clarity measurements vary from day-to-day and year-to-year, managers and scientists remain focused on long‐ term trends as an indicator of the lake’s health. Measurements show Lake Tahoe’s annual clarity has plateaued over the past 20 years.

Despite this progress, summer clarity continues to decline by over a half‐foot per year.

Geoffrey Schladow, UC Davis TERC director, said he doesn’t think there is a link between the increased tourism last year due to COVID, and last summer’s low clarity readings, especially since the decline in summer clarity has become a trend.

They are, however, studying the impacts smoke from wildfires has on clarity.

“I don’t think that would have accounted for last year but what we’re trying to look at is the impact of the smoke cumulatively as we keep having these large megafires,” Schladow told the Tribune. “That’s ongoing research to look at how that might affect things in the long-term.”

A recent review of clarity data by the Tahoe Science Advisory Council reaffirmed the understanding of main drivers of clarity loss. The council commissioned a panel of scientists from regional academic and government research institutions which concluded that fine sediment particles and algae continue to be the dominant variables affecting Tahoe’s clarity. They recommended that water quality agencies continue to focus on reducing fine sediment and nutrient loads.

Past UC Davis research and the council’s report pointed to several other factors affecting Tahoe’s famed clarity. Climate change is altering precipitation and snowmelt patterns, increasing the temperature of the lake, and impeding deep lake mixing. Such mixing in late winter can bring cold, clear water up from deep in the lake which improves clarity. In 2020, the mixing was extremely shallow and contributed to the lack of improvement.

“Adaptive management is crucial when confronting evolving threats like climate change, invasive species, and expanding visitation rates in the Tahoe Basin, but it is an approach that requires targeted data to assess response to changing conditions and management actions,” Alan Heyvaert, past Tahoe Science Advisory Council co-chair and Desert Research Institute associate research professor said in a press release.

Despite the increasing severity of the impacts of climate change, Schladow is hopeful they can make a positive impact on clarity.

“I know it sounds wishy-washy to say ‘we know more so we can do better,’ but it’s true,” Schladow said. “We know so much more now and I think what’s important is that management agencies are with us on this. They realize the climate is changing, everything is changing, the lake itself is changing and so everybody seems to be moving forward with a similar level of understanding and definitely with the same end goal which is to improve clarity, not to just maintain it.”

The Total Maximum Daily Load program is the multi-agency collaborative to control Tahoe’s main pollutants through erosion control projects, roadway improvements and road maintenance. The water quality restoration goals of the Lake Tahoe Environmental Improvement Program fit within the TMDL and TRPA has spent more than $1 million in watershed and water quality improvement projects.

“Both of the main drivers, sediment and nutrients, have been the target of land use policies in the basin for many years and a lot of progress has been made,” said Jeff Cowen, public information officer for TRPA. “Private property owners have completed erosion control BMPs on tens of thousands of parcels and local governments and state agencies have upgraded hundreds of miles of roadway to cut pollutants.”

Work on transportation and road rehabilitation helps cut the amount of nitrogen from in-basin transportation emissions and combats fine sediment from entering the lake because of declining road conditions.

“Restoring streams, marshes and wetlands is the other major action toward restoring lake clarity,” Cowen said. “There have been dozens of stream restoration projects and the keystone marsh project on the Upper Truckee River is underway. To restore some function, we also have incentive programs that make restoring and removing existing development in stream environment zones an attractive option for private investors.”

While these big picture projects can help improve clarity, Schladow also said educating locals and visitors can make an impact as well.

“I think people can stay vigilant and stay aware of what you’re doing,” Schladow said. “I mean, we’re the biggest problem here, and I’m including myself… everything has an impact so the idea of let’s try to have the least impact possible while enjoying the lake.”

To learn more about the clarity report, visit https://tahoe.ucdavis.edu/secchi.