Lake Tahoe's clarity drops five feet

Warmer water led to a loss in clarity of Lake Tahoe in 2015, according to data released Tuesday by the UC Davis Tahoe Environmental Research Center and the Tahoe Regional Planning Agency. The average annual clarity (measurements from both summer and winter) is at 73.1 feet, a 4.8 foot decease from the previous year.

This isn't all bad news though, the clarity is still over nine feet greater than the lowest recorded average of 64.1 feet in 1997.

Researchers lower a 10-inch wide disk called a Secchi into the lake, and researchers measure the depth of its visibility.

According to a report released April 19, 2016 by UC Davis, the declines are not considered to be part of a long-term trend. They are part of the year-to-year variability that has always characterized conditions at the lake. The record indicates that Lake Tahoe’s long-term trend of decline ended about 15 years ago. Since then, clarity has hovered around a value of 71 feet.

“It is commonly believed that drought years produce clearer water conditions, but the reality is more complicated than that,” said Geoffrey Schladow, director of the UC Davis Tahoe Environmental Research Center.

The annual precipitation for 2015 was 18 inches, only slightly lower than the previous year. However, the fraction of precipitation that fell as snow was only 5.3 percent, the lowest recorded measurement in 105 years.

“In 2015, the runoff water into the lake was warmer than the previous year, on account of the low snow-to-rain ratio,” Schladow said. “As a consequence, fine particles flowed in closer to the surface, where they impacted clarity, rather than plunging to the deeper parts of the lake.”

The 2015 clarity level is the average of 30 individual readings taken from January through December 2015. The highest value recorded in 2015 was 86.5 feet on April 22, and the lowest was 59.9 feet on Feb. 3.

Winter clarity last year declined by 7.6 feet. The winter average of 71.6 feet was above the worst winter average, 66.6 feet, seen in 1997.

During most years, summer clarity is poorer than winter clarity, but 2015 was an exception. Summer clarity averaged 73 feet, a 3.7 foot decline over the preceding year. The lack of deep mixing during the winter resulted in very little of the pristine, deep water being brought up to the lake surface to dilute the clarity-reducing contaminants. Despite this, the long-term decline in summer conditions is still a major concern.