Keeping Tahoe very blue: Lake Tahoe’s clarity improved by 10.5 feet last year

Lake Tahoe’s water clarity improved in 2018 mainly due to a typical winter season and water flow on rivers and streams, according to a new study.

The annual report by UC Davis researchers found the average clarity level in 2018 was 70.9 feet, a 10.5-foot increase from 2017. Two years ago clarity had dropped to a record low, partially due to record-breaking precipitation and unusually warm lake temperatures.

Scientists measure clarity by lowering a 10-inch white disk into the water and noting the depth at which it become invisible. They took 26 readings from January through December in 2018.

Clarity is typically best in fall and winter. The winter (December to March) average was 73.5 feet, with one reading in March measuring more than 100 feet. The lowest value was 50 feet in July 2018, while the summer average (June to September) was 61.7 feet.

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A number of factors impact the clarity of the lake, but a primary influence is the spring runoff, the water that pours into the lake as the snowpack melts. While a winter marked by excessive snow might be good for filling the lake with water, a massive snowpack sends rivers roaring in the spring and washes more sediment into the lake. Last year’s runoff flows were significantly less than those in the record-wet year of 2017.

“In 2018, Lake Tahoe’s clarity regained the expected seasonal patterns that were disrupted by the extreme conditions of the previous year,” Geoffrey Schladow, the director of the Tahoe Environmental Research Center and a professor of engineering at UC Davis, said in a statement. “Clarity was expectedly at its lowest point in summer, and winter and fall had the highest values. Devising strategies to improve summer clarity in the long term is a high scientific priority.”

Lake Tahoe’s clarity is legendary and stuns the 3 million visitors who gaze across its waters every year. The waters grew murkier from the late 1960s through the early 2000s due to algae growth and the presence of very fine particles. In the past decade, the rate of deterioration has lessened as public and private groups find ways to reduce stormwater pollution from roads and urban areas. Restoring meadows can filter out sediment from the runoff before it pours into the lake, according to the UC Davis study.

“We are thrilled to see Lake Tahoe’s clarity improving from the all-time low of just 60 feet in 2017,” said Darcie Goodman Collins, the CEO the League to Save Lake Tahoe, one of the groups helping to preserve the lake’s clarity.

Collins continued in a statement, “These results encourage us to continue restoring critical habitat and improving our urban areas to keep pollution from entering our lake.”

This post was originally posted at https://www.sfgate.com/bayarea/article/Lake-Tahoe-clarity-study-UC-Davis-2018-13883594.php.

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