UC Davis environmental scientists say Lake Tahoe’s clarity “dramatically” improved in 2018, after 2017 represented the worst annual clarity the university has ever recorded at the lake, according to a report released Thursday.
The measuring process for Tahoe clarity is simple: A small white “dinner plate”-shaped device called a Secchi disk is lowered from a boat into a set point in the lake, and scientists with the UC Davis Tahoe Environmental Research Center (TERC) make note of how deep the Secchi disk remains visible.
Researchers do this about two dozen times each year and compute water clarity seasonal averages, for winter and summer, as well as annual averages. After 2017 reached a rock bottom clarity reading of 60.4 feet, the 2018 average soared back to 70.9 feet, the best it’s been since 2015 (73.2 feet).
Clarity peaked in the first year TERC started measuring, at 102.4 feet in 1968. The annual clarity, by foot, gradually dipped - remaining roughly in the 80s through most of the 1970s, to the 70-to-80 foot range in the 1980s.
From the early 1990s to today, it has ranged from 60 to 78 feet.
“The goal is to restore lake clarity back to its historic level of nearly 100 feet,” a news release accompanying a Thursday report by TERC said.
Lake Tahoe clarity depends largely on algae levels, which correlate positively with stormwater pollution and warm weather. Pollution reaches the lake from nearby roads and urban areas by way of surrounding tributaries, the news release said.
The university’s research report showed sediment washing into Lake Tahoe from its major tributaries during spring 2018 measurements only totaled 10 to 25 percent of sediment levels observed the previous spring.
“Seasonal weather extremes will most likely drive greater swings in clarity from year to year in the future, so it’s imperative we continue to invest in the lake’s restoration to combat new and emerging threats,” Joanne S. Marchetta, executive director of the Tahoe Regional Planning Agency, said in an accompanying statement.
Clarity varies by season, as warmer weather and direct sunlight stimulate algae growth. Tahoe is clearest during fall and winter, and least clear during the summer. The worst summer average for Secchi disk visibility came in 2011, at 51.5 feet.