Drought, then flood, muddies Lake Tahoe | RecordCourier.com

Drought, then flood, muddies Lake Tahoe

Staff Reports

While acknowledging that a record-breaking winter after several years of drought was in part to blame, scientists were surprised at how much clarity Lake Tahoe lost during 2017.

The average clarity for the lake during the year was 59.7 feet, which was nearly 5 feet worse than in 1997, when the first Tahoe Summit set the goal to improve its clarity.

"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 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. Measurements for 2018 have already shown a large improvement that are more in line with the long-term trend."

Up until 1997, clarity was steadily improving, in part due to the drought, which meant less runoff to wash sediment and pollutants into the Lake.

When Europeans first arrived in the central Sierra, Lake Tahoe was so clear they reported being able to see 100 feet to the bottom.

Last year was the wettest year on record in Douglas County with 20 inches of precipitation falling during the calendar year. It was the seventh wettest water year on record, according to UC Davis researchers.

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"While both average winter and average summer clarity values decreased in 2017, neither were the lowest on record," researchers said. "The average winter value was 76.4 feet, which is 10.8 feet above the worst recorded level of 65.6 feet in 1997. The average summer value was 53.5 feet, which is 3 feet above the lowest recorded level of 50.5 feet in 2008. However, fall readings for clarity were the lowest on record, causing the overall average to drop."

Work is ongoing to reduce the amount of stormwater pollution from roads and urban areas around the Lake.

"Partners are also working to restore natural wetlands and meadows that were displaced by past development and play an important role in filtering water before it enters the lake," researchers said. "That work is keeping tens of thousands of pounds of fine sediment particle pollution out of Lake Tahoe each year, and the lake has been responding with an improved clarity trend over the past 20 years. The goal is to restore lake clarity back to its historic level of nearly 100 feet."