Report: Scientists say Lake Tahoe's famed water clarity shows alarming downturn

The UC Davis Tahoe Environmental Research Center released findings Wednesday from its annual Lake Tahoe Clarity Report, which showed In 2019 the lake's famed clarity decreased nearly 8 feet from the previous year’s dramatic 10-foot improvement. The average annual value in 2019 was 62.7 feet. The lowest value was recorded in 2017, when clarity was 60 feet.

The average Lake clarity for 2019 was 62.7 feet, the second-smallest annual average depth recorded since systematic annual measurements began in 1968, according to the report, which can be found here.

Clarity is measured as the depth to which a 10-inch white disk, called a Secchi disk, remains visible when lowered into the water. In 2019, scientists took 28 individual readings at the long-term index station maintained by UC Davis.

This drop in clarity from 70.9 feet in 2018 marks a significant downturn and a warning that large annual swings in variability should be expected as the impacts of climate change on the Tahoe-Truckee watershed become more extreme, according to the environmental organization League to Save Lake Tahoe.

“On its face, this data is startling, but not unexpected. After an encouraging 10-foot gain in clarity from 2017 to 2018, the losses from 2018 to 2019 reveal how strongly Tahoe’s blue is tied to the effects of the climate crisis and influenced by the Lake ‘turning over’ or mixing all the way to the bottom," said League to Save Lake Tahoe CEO Darcie Goodman Collins, PhD in a news release. "It also spotlights the need for everyone of us — public agencies, environmental organizations, local businesses, visitors and residents — to do all we can to Keep Tahoe Blue."

UC Davis has conducted continuous monitoring of Lake Tahoe since 1968, when a Secchi disc could still be seen a full 102 feet below the surface. This research has informed policymakers and stakeholders on management strategies to protect the lake and stabilize its decline in clarity.

“Understanding why summer clarity continues to decline is our highest priority, and we continue to work closely with the science community to understand how to reverse that decline,” said Joanne S. Marchetta, executive director of the Tahoe Regional Planning Agency. “We remain committed to the lake’s restoration in the face of the urgent threats of climate change and invasive species.”

This year’s results send a clear message that large swings in Lake clarity year-over-year are the new norm as climate change exerts its influence here in the Basin, said Collins of The League to Save Lake Tahoe in a press statement.

More intense and sporadic weather events, prolonged dry periods, a shift in precipitation to more snow than rain and warming surface temperatures all impact how deeply we can see into Tahoe’s waters, Collins said.

What’s more troubling is that climate change intensifies all the other threats facing Tahoe, such as the spread of aquatic invasive species, hazardous algal blooms, the risk of wildfire and stormwater pollution, said the League's press statement.

"While Mother Nature got a well-deserved break when shelter-in-place orders were in effect, climate change won’t go away that easily," according to the League's press statement. "The quick recovery we witnessed in air quality and wildlife activity over the last few months should give us hope that by reducing global emissions, the ecosystem will bounce back and Tahoe’s clarity will improve. On a local level, it’s crucial that we are aware of the forces damaging Lake clarity and are actively working to minimize their impacts here in the Basin."

More than 80 organizations, including government agencies and research institutions, are working in collaboration to address environmental impacts to Lake Tahoe’s fragile ecosystem.

State, local and federal agencies, together with the private sector and other stakeholders, are working collaboratively with scientists to improve Lake Tahoe’s water clarity and ecological health under a bi-state clarity restoration program and the basin’s Environmental Improvement Program.

While the impacts of climate change and watershed contaminants have long been focal points of research, new research is exploring the impacts of the invasive Mysis shrimp on the native food web.

“With lake warming impacting the recovery of Lake Tahoe’s clarity, we’re looking for ways that can mitigate climate impacts. Approaches such as the removal of the invasive Mysis shrimp are showing great promise for clarity improvement in the next few years,” said Geoffrey Schladow, director of the UC Davis Tahoe Environmental Research Center. “The environmental services that the natural Tahoe ecosystem provided are now being better recognized.”