Lake Tahoe’s cobalt-blue waters were about as clear in 2021 as they were in 2020. But a broader look at clarity metrics shows there’s no trend for steady improvement in clarity over the past 20 years. The lake has also not fully recovered from a spike in fine particles that poured into its waters after the extremely wet year of 2017.

That’s according to data collected through 2021 by the University of California, Davis, Tahoe Environmental Research Center. UC Davis has measured clarity and other health indicators at Lake Tahoe since 1968, helping inform policy makers and stakeholders about lake protection strategies and stabilizing the decline in clarity that dates back to the development boom of the region in the 1960s.

(Download infographics and clarity charts in our press kit.)

Average annual lake clarity from 1968 to 2021. (UC Davis Tahoe Environmental Research Center)

Recent years have presented evolving and new threats to Lake Tahoe, as global warming, floods, droughts, and wildfires impact the lake in ways that are not fully understood.

“The lake itself is changing internally, and the external inputs that impact lake clarity and health are changing along with it,” said Geoffrey Schladow, director of the UC Davis Tahoe Environmental Research Center. “We are working with other Lake Tahoe researchers and with partner agencies to not only track clarity, but also to adapt management approaches to improve clarity in the years to come.”

A confusing scheme

Lake Tahoe’s average annual clarity in 2021 was 61 feet compared to 63 feet in 2020. Summer measurements were 54.8 feet, while winter averages were 71.9 feet.

While clarity during the winter months is consistently better than during the summer, the trend over the past two decades indicates that neither summer nor winter clarity levels improve over time.

Particle problems

Decades of research led to the development of Lake Tahoe’s Total Maximum Daily Load, or TMDL, the science-based plan to restore the lake’s historic clarity. TMDL science has identified fine particles and tiny algae as playing an important role in determining lake clarity. Currently, these are responsible for up to 70% of clarity loss.

Public and private investments in improving water quality over the past 25 years have significantly reduced fine particulate matter and algae-feeding nutrients entering Lake Tahoe, and TMDL pollution load reduction targets are achieved.

However, fine particles have remained elevated since 2017, when unprecedented winter storms contributed to Tahoe’s worst clarity on record. Fine particles in Tahoe streams quadrupled that year and have remained above the historical average since then. Fine particles in the lake have also increased and have not yet recovered to previous levels.

UC Davis scientists chart the average concentration of fine particles in Lake Tahoe monthly. Note the peak in 2017, with high average concentrations to the present day. (UC Davis Tahoe Environmental Research Center)

Schladow notes that some of the lake’s particle readings were likely influenced by smoke deposits from past years of wildfires that blanketed the basin. The precise role of wildfires on the clarity and overall health of the lake is the subject of a Tahoe Scientific Advisory Board and multi-institutional study, the results of which are expected later this year.

“Extreme weather events and changing lake dynamics make our investments in water quality even more important,” said John Hester, acting executive director of the Tahoe Regional Planning Agency. “Through strong partnerships within Tahoe’s scientific community, we will continue to improve our understanding of how climate change could impact the lake’s clarity restoration plan.”

A clear history

Clarity is measured as the depth to which a 10-inch white disc, called a Secchi disc, remains visible when lowered into water. In 2021, UC Davis scientists took 22 individual readings at the Lake Tahoe Long-Term Index Station. View historical clarity readings from 1968 to 2021 at tahoe.ucdavis.edu.

Using a range of technologies beyond the Secchi disc, researchers continue to refine their understanding of the interactions of lake physics and ecology to determine the evolutionary causes of change in clarity. In 2021, underwater autonomous gliders were added to the instruments now focusing on changes in clarity in the lake.

Katie Senft, a researcher at the UC Davis Tahoe Center for Environmental Research, lowers a Secchi disk to measure lake clarity in June 2021. Center researchers make dozens of such measurements each year. (Alison Toy/UC Davis TERC)

The states of California and Nevada, which share a border at Lake Tahoe, are actively working to restore the lake’s clarity to its historic 97.4 feet.

More than 80 organizations, including government agencies, nonprofits, and research institutes, are working collaboratively with scientists to improve Lake Tahoe’s water clarity and ecological health as part of the program. Tahoe Environmental Improvement, or EIP, which is one of the most comprehensive landscape-scale restoration programs. programs in the country. IEP partners help achieve TMDL reduction goals by reducing pollution through improved road maintenance and erosion control on roads and private property.

Science partners will continue to research changes in climate and clarity in Lake Tahoe and inform decision makers on strategies to restore the lake’s historic clarity. The Tahoe Science Advisory Council, an independent group of research institutions, including the UC Davis Tahoe Environmental Research Center, also conducts an annual lake clarity analysis. The 2021 Clarity Conditions Council report is available at tahoesciencecouncil.org.

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