Lake Tahoe’s clarity improved in 2020 — but not by much, according to researchers

Lake Tahoe’s clarity improved in 2020, but only by a very small margin, according to new data from the UC Davis Tahoe Environmental Research Center.

The average clarity of the lake was 62.9 feet, hitting its peak in February 2020, when it was deeper than 80 feet. And its clarity was at its worst in mid-May, when it measured at slightly more than 50 feet deep.

In 2019, the average clarity was slightly shallower at 62.7 feet.

UC Davis researchers have measured the clarity of Lake Tahoe since the 1960s. To do it, they use a 10-inch white disk, called a Secchi disk, and measure the depth at which it remains visible when lowered through the water. Clarity of the lake changes day to day, so researchers use average depth over a year to gauge the health of the lake.

“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,” the center said in a news release.

The main reason the lake loses clarity is because of larger amounts of sediment and algae in the lake, the report said. Data show that the lake’s clarity has historically been about 100 feet, but clarity has plateaued in the last 20 years.

“Climate change is altering precipitation and snowmelt patterns and increasing the temperature of the lake and impeding deep lake mixing,” the center said. “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.”

This story was originally published July 9, 2021 at 5:00 AM.