The famed crystal blue waters of Lake Tahoe are muddier than ever, researchers say.
The combination of extreme drought and heavy rainfall in 2017 washed 12,000 tons of sediment into the lake, causing the 2017 average clarity levels to drop to the lowest on record, according to a new clarity report by the Tahoe Environmental Research Center at UC Davis.
Clarity is measured by lowering a 10-inch white disk, called a Secchi disk, into the water until it is no longer visible, the report said.
The average clarity level was 59.7 feet in 2017, which was a 9.5-foot decrease from 2016. The average for 2017 also surpassed the previous record low in 1997 in which 5,000 tons of sediment flowed into the lake from mountain streams, said Geoffrey Schladow, director of the Tahoe Environmental Research Center.
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Summer temperatures at Lake Tahoe were also the warmest on record, and research "has predicted that warming lake temperatures would hold fine sediment particles closer to the surface longer, reducing clarity," according to the report.
However, Schladow called the all-time low in average clarity "a freak confluence" rather than the new normal.
During the drought, only about 1,000 tons of sediment was flowing into Lake Tahoe per year, he said. So, the record rainfall in 2017 washed five years of material into the lake.
Researchers continue to monitor the lake's clarity to see if normal trends resume. So far, clarity improved to 65 feet last week, Schladow said. "It's right where we would expect it to be if not a little bit better."
Work is being done to keep sediment out of Lake Tahoe, the report said. The goal is to reach lake clarity of 100 feet.
The low clarity level in the lake has no major impact on recreational activities, Schladow said. "People who come still see this incredible blue lake."
Molly Sullivan: 916-321-1176, @SullivanMollyM