How clear is Lake Tahoe? Annual report shows 2020 low clarity at 50 feet, high at 80 feet

LAKE TAHOE, Calif./Nev. - Lake Tahoe’s water clarity measurements, which are indicators of the health of the watershed, averaged 62.9 feet through 2020, the UC Davis Tahoe Environmental Research Center (TERC) and the Tahoe Regional Planning Agency (TRPA) announced today.

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.

Researchers from UC Davis have been recording lake clarity since the 1960s by using a 10‐inch white disk, called a Secchi disk. They record how deep they can see the disk from the surface.

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.

Lake Tahoe is known around the world for its water clarity and cobalt blue color. Historically, clarity averaged about 100 feet. A development boom in the mid‐20th century brought about unintended environmental impacts, including reduction of the lake’s pristine clarity. For decades, researchers have been documenting changes in the lake and the research has informed policymakers and stakeholders on management strategies to protect the lake and stabilize its decline in clarity.

In 2020, UC Davis scientists took 27 individual Secchi readings at Lake Tahoe’s long‐term index station. Using technology beyond the Secchi disk, researchers continue to refine their understanding of lake physics and ecology to determine the evolving causes of clarity change.

“Regaining Lake Tahoe’s water clarity is a commitment we all share, and together we are making a difference,” said Joanne S. Marchetta, executive director of the Tahoe Regional Planning Agency. “While the long‐term clarity trend shows we are on the right track, we need to be vigilant about external factors and the role climate change and other threats are playing.”

“The latest Clarity Report shows us once again that we need to take bold action to slow climate change’s negative impact on Lake Tahoe’s clarity, ecology and natural beauty," said the League to Save Lake Tahoe CEO Darcie Goodman Collins, Ph.D. "Tahoe’s surface waters are getting warmer, acting like a seal that prevents mixing between layers in the Lake’s water column. So when fine-grained particles of dirt and pavement from overused roads flow into the Lake during a rainstorm, they get trapped near the surface, hurting the Lake’s clarity. Warm, nutrient-filled waters are also the perfect habitat for harmful aquatic invasive species to spread."

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.

"While there is a good understanding of how fine clay particles and tiny algal cells reduce clarity, the biggest challenges are in reducing their presence in the surface water,” said Geoffrey Schladow, UC Davis’ Tahoe Environmental Research Center director. “Here climate change, and in particular the warming of the surface water, is exerting an undue influence.”

"If Tahoe was untouched by humans, the Lake’s natural pollution filters – its marshes, meadows and wetlands – would strip those fine-grained particles out of the water before reaching the Lake. But decades of short-sighted development paved over those natural filters. And Tahoe’s roads are constantly bombarded with traffic, chewing up the pavement into fine particles that enter the Lake as stormwater pollution," said Goodman. "To Keep Tahoe Blue – and therefore clear – the League is working across the public and private sector to: restore Tahoe’s wetlands to revitalize their natural pollution-fighting function and build Tahoe’s resilience to climate change; put aquatic invasive species in check, so Tahoe’s shallows don’t turn into a murky, green mess; repair, repave or replace Tahoe’s ailing roads with modern infrastructure on a Basin-wide scale; and create transportation solutions in Tahoe to break the dependence on private car travel and stop stormwater pollution before it starts."

Dr. Schadlow will present his annual State of the Lake report on August 12, 2021 from 12:00 p.m. - 1:00 p.m. It will be presented via Zoom. For more information, visit https://tahoe.ucdavis.edu/events/2021-state-lake-report.