Report: Lake Tahoe clarity improves by over 10 feet in 2018

Lake Tahoe's renowned clarity experienced a dramatic improvement in 2018 compared to the previous year — a fact researchers attribute to a more mild winter.

According to data released by the UC Davis Tahoe Environmental Research Center on Thursday, Tahoe's annual clarity value improved to 70.9 feet in 2018. The number represents a 10.5-foot increase over the 2017 value.

The improvement was somewhat expected.

Clarity can swing dramatically from day to day and year to year based on a multitude of factors including heavy precipitation, which increases streamflow and leads to more sediment flowing into the lake.

As researchers noted in last year's annual report, the convergence of extreme events — years of drought followed by one of the largest winters on record in 2016-17 — had a dramatic impact on clarity in 2017. That year researchers reported a decrease of 9.5 feet.

The major swings in clarity make long-term averages especially important.

The five-year average lake clarity is currently 70.3 feet, an increase of almost a foot from the previous five-year running average.

“In 2018, Lake Tahoe's clarity regained the expected seasonal patterns that were disrupted by the extreme conditions of the previous year,” TERC Director Geoffrey Schladow, a professor of engineering at UC Davis, said in a press release.

Clarity is typically lowest in the summer after spring runoff brings fine sediment particles and nutrients to the lake, and warmer temperatures and more sunlight stimulate algae growth, according to TERC. The lowest individual value was 50 feet in July 2018 , while the mean summer (June-September) clarity value was 61.7 feet.

Spring runoff flows in 2018 were far lower and occurred earlier than in 2017. The quantities of suspended sediment washed into the lake from the major tributaries were 10-25% of the 2017 values, according to TERC.

The biggest driver of summer clarity was the occurrence of high concentrations of the phytoplankton species Cyclotella gordonensis. This microscopic algae has become more prevalent in Lake Tahoe in recent years. Its very small size makes it a potent scatterer of light and a major factor in annual Secchi — a 10-inch white disk used to measure clarity — depth.

Clarity tends to improve during the fall and winter months. The average winter (December-March) clarity was 73.5 feet, with one reading in March 2018 exceeding 100 feet of clarity.

“Clarity was expectedly at its lowest point in summer, and winter and fall had the highest values. Devising strategies to improve summer clarity in the long term is a high scientific priority,” Schladow said.

Thursday's news was cheered by environmental advocacy groups.

“We are thrilled to see Lake Tahoe's clarity improving from the all-time low of just 60 feet in 2017,” said CEO of the League to Save Lake Tahoe Darcie Goodman Collins. “These results encourage us to continue restoring critical habitat and improving our urban areas to keep pollution from entering our lake. We need to ensure Lake Tahoe is resilient to a changing climate where periods of drought followed by sporadic intense storms will be more common if we hope to Keep Tahoe Blue.”

Efforts to combat loss of clarity have been ongoing for decades at the lake.

The ultimate goal is to restore lake clarity back to its historic level of nearly 100 feet.

Among the efforts underway, the Tahoe Science Advisory Council — a body comprised of scientists from UC Davis, UC Santa Barbara, the University of Nevada, Reno, the Desert Research Institute, the U.S. Geological Survey and the U.S. Forest Service — is currently finalizing a new “science to action plan.”

The plan, according to TERC, will identify key modeling and monitoring needs to more fully account for climate change impacts on lake clarity and ecological processes.

“Research shows Lake Tahoe and other inland water bodies are warming faster than the oceans and atmosphere,” Joanne Marchetta, executive director of the Tahoe Regional Planning Agency, said in the press release. “Seasonal weather extremes will most likely drive greater swings in clarity from year to year in the future, so it's imperative we continue to invest in the lake's restoration to combat new and emerging threats.”