Famous clarity of Lake Tahoe sharply drops

The epic clarity of Lake Tahoe was measured at an average of 59.7 feet in 2017, a 9.5 foot drop from the previous year and 10.3 feet lower than the five-year average according to the annual report compiled by the Tahoe Environmental Research Center at the University of California, Davis.

This measurement surpasses the previous lowest level of 64.1 feet which was recorded in 1997, but far below the record high of over 100 feet in the mid-1960s.

“In 2017, Lake Tahoe’s low clarity was primarily the result of two extreme climatic and hydrologic events—a perfect storm, so to speak,” said TERC Dr. Director Geoffrey Schladow, a professor of engineering at UC Davis. “The combination of arguably the most extreme drought period ending with the most extreme precipitation year produced the low clarity values seen.But this does not reflect the long-term trend, and measurements for 2018 have already shown

"The record-level low for Lake Tahoe’s clarity is alarming news," said Jesse Patterson, deputy director for the League to Save Lake Tahoe. "That said, it’s not surprising, considering how much the Lake has been warming in recent years. Tahoe has also been experiencing more frequent extreme weather conditions, which may be the new normal at Lake Tahoe as our climate changes."

Clarity is typically at its worst in the summer and tends to improve during the fall and winter months. In 2017, a year when nearly every corner of the globe experienced extreme weather events, that pattern diverged with the worst clarity extending through the fall.

From October 2016-September 2017, California had its second wettest year on record, and it was Nevada's seventh wettest. The Lake Tahoe Basin saw up to 300 percent of its normal precipitation That, coupled with an early winter storm in November 2017, added an unusual amount of sediment to the lake, affecting clarity.

Most of the sediment load in the low-clarity year of 1997 came in January with the peak flow. But 2017 produced a far greater load of sediment than the lake experienced in 1997, and those loads came more frequently throughout the year.

The year 2017 marked the end of California’s worst drought in at least 1,200 years, followed by one of the wettest winters on record, when more than 50 atmospheric rivers emptied over the region.

"Global warming is expected to make extreme weather fluctuations more common at Tahoe," said Patterson. "Also, in the past four years, Lake Tahoe’s water has been warming at 10 times its historic trend."

Dozens of public and private partners at Lake Tahoe are working to reduce stormwater pollution from roads and urban areas and to restore streams to prevent erosion that harms lake clarity. Partners are also working to restore natural wetlands and meadows that were displace by past development and play an important role in filtering water before it enters the lake.

“Scientific research has predicted a changing climate could affect lake processes, and this likely means we can expect more swings in clarity from year to year,” said Joanne S. Marchetta, executive director of the Tahoe Regional Planning Agency, which funded the analysis. “In response, we've been accelerating our investments in projects to bolster resiliency to emerging threats like extreme weather. It's important to continue to make those smart investments for the long-term health of the lake and its environment.”