Lake Tahoe clarity in 2014 best in more than decade

An aerial photo of the MS Dixie II and the Zephyr Cove area is seen Thursday.

An aerial photo of the MS Dixie II and the Zephyr Cove area is seen Thursday.

While the drought was bad news for ski resorts and irrigators, it may have contributed to the good news for Lake Tahoe’s clarity.

Researchers from the University of California, Davis, reported clarity levels at Lake Tahoe in 2014 showed the biggest improvements in more than a decade.

The improvements are in part due to continuous work from the Lake Tahoe community to lower pollutants to the lake. They were also influenced by the drought, as reduced precipitation meant fewer contaminants flowed into Lake Tahoe, particularly during the summer, when clarity levels were the highest recorded since 2002.

Data released Tuesday by the UC Davis Tahoe Environmental Research Center and Tahoe Regional Planning Agency reported the average annual clarity level for 2014 at 77.8 feet. That’s the depth at which a 10-inch white disk, called a Secchi disk, remains visible when lowered in water.

This represented a 7.5 foot increase over the previous year and was almost 14 feet greater than the value of 64.1 feet in 1997, when the lowest average clarity value was recorded.

“While these latest data are very reassuring, they should not be interpreted as victory in our joint restoration efforts,” said Geoffrey Schladow, director of the UC Davis Tahoe Environmental Research Center. “Complete restoration is still decades away, and some of the greatest challenges still lie ahead of us. We are enduring drier and warmer conditions than we have ever encountered, and the restoration consequences of that are still far from understood.”

The clarity level this year is the average of 28 individual readings taken throughout the year. The highest individual value recorded in 2014 was 93.5 feet on July 7, and the lowest was 57.4 feet on Sept. 16.

Researchers provided measurements for both winter (Dec.–March) and summer (June–Sept.) months. Winter clarity last year improved by 1 foot, continuing the long-term pattern of improvement since 1997. The winter average of 78.7 feet was well above the worst winter average, 66.6 feet, seen in 1997.

The largest improvement was seen in summer clarity, where there was a 13-foot improvement over the preceding year. At 78.7 feet this is the highest value since 2002.

The long-term trend in summer clarity is complex, and is the result of many factors, including the amount of precipitation in the basin and the depth to which the lake mixed the previous winter.

In 2014, the ongoing drought produced small inflows, which transported fewer contaminants into Lake Tahoe. The warmer conditions also resulted in shallow mixing. Years of shallow mixing are often years of high clarity.

Water clarity measurements have been taken continuously since 1968, when the Secchi disk could be seen down to 102.4 feet.

While the average annual clarity in the past decade has been better than in preceding decades, it’s still short of the clarity restoration target of 97.4 feet set by federal and state regulators.

More information about environmental factors affecting Lake Tahoe will be included in the 2015 State of the Lake Report, expected this summer.


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