Lake Tahoe clarity decreased nearly 10 feet in 2017

Citing the convergence of drought, record precipitation and warm temperatures, researchers announced Wednesday that Lake Tahoe’s famed clarity declined 9.5 feet in 2017.

The numbers from the UC Davis Tahoe Environmental Research Center (TERC) follow a 3.9-foot decrease in average annual clarity in 2016. The decrease in 2017 dropped the average annual clarity level to 59.7 feet.

Although the measurement plummeted past the previous lowest recorded average of 64.1 feet, which was recorded in 1997, officials pointed out clarity can swing greatly from year to year and from season to season.

The five-year average lake clarity is approximately 70 feet.

“In 2017, Lake Tahoe’s low clarity was primarily the result of two extreme climatic and hydrologic events — a perfect storm, so to speak,” TERC Director Geoffrey Schladow, a professor of engineering at UC Davis, said in a statement. “The combination of arguably the most extreme drought period ending with the most extreme precipitation year produced the low clarity values seen. Measurements for 2018 have already shown a large improvement that are more in line with the long-term trend.”

The impact of years of drought followed by one of the wettest winters on record is reflected in last year’s seasonal clarity shifts, or lack of shifts, researchers said.

Typically clarity is worst in the summer months before improving in the fall and winter months. While clarity through mid-March fared better than in previous years, it failed to improve in the fall and winter.

Researchers say this is due, in part, to the volume and increased frequency that sediment was dumped into the lake. In 1997, which also saw a heavy-precipitation winter, peak sediment flow came in January.

That was not the case in 2017, which marked the end of California’s worst drought in at least 1,200 years.

According to the research center, “2017 produced a far greater load of sediment than the lake experienced in 1997, and those loads came more frequently throughout the year.”

That point was further illustrated by the fact algal concentrations — another leading cause of reduced clarity — weren’t significantly higher in 2017 than other years.

Researchers also cited warming temperatures as an ingredient in the low-clarity cocktail. According to the research center, 2017 summer temperatures were the warmest on record at Lake Tahoe — nearly 3 degrees warmer than in the previous three years.

Warming lake temperatures can hold fine sediment particles closer to the surface longer, reducing clarity, according to previous research.

Reaction

Wednesday’s report sparked a number of reactions from environmental and elected officials, most of whom noted the alarming figure with the caveat of the conditions that caused it.

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

In a joint statement, California Natural Resources Secretary John Laird and Nevada Director of Conservation and Natural Resources Bradley Cowell called for further analysis of the data.

“According to the latest data, one of the most iconic indicators of Lake Tahoe’s health — lake clarity — registered its lowest recorded annual level in 2017. While annual clarity declines are not unusual, the record decline experienced last year warrants an in-depth review to further understand the causes and impacts, and to help ensure the 2017 decrease is an anomaly, and not a trend.”

In recognizing ongoing impacts attributed to climate change, several officials noted the recent and ongoing efforts to improve clarity.

“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,” Joanne Marchetta, executive director of the Tahoe Regional Planning Agency, which funded the analysis, said in a statement. “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.”

Sen. Dianne Feinstein (D-Calif.) said the numbers should be a catalyst for further investment.

“This report serves as a stark reminder about the importance of conservation and restoration efforts,” Feinstein said in a statement. “Without that progress, the lake would already have been lost. We must continue investing in those efforts if we’re going to save Lake Tahoe.”