Annual lake clarity reading: A slight improvement
An improvement of 3 feet may not be a big deal, but at least Lake Tahoe’s clarity got better in 1999 and not worse.
The average depth at which a white disk dropped into Lake Tahoe could be seen in 1999 was 69 feet, according to the University of California, Davis.
That number is an improvement over the last two years; however, 1997, 1998 and 1999 are the three worst years since the U.C. Davis Tahoe Research Group started measuring Tahoe’s clarity in 1968.
The average depth of clarity often fluctuates substantially from year to year, and researchers are quick to emphasize that a single year’s measurement isn’t as important as the long-term trend.
“It’s nothing to get excited about either way,” said Bob Richards, U.C. Davis researcher. “It’s a little bit better, but you just have to place it in with the rest of the long-term information.”
Tahoe’s famously clear water has become increasingly murky, losing an average of more than a foot of clarity a year for more than three decades. From the 37-foot aluminum research vessel the John Le Conte, U.C. Davis scientists drop a Secchi disk, much like a white dinner plate, into Tahoe’s depths several times a month every year. The 1999 average is based on 35 measurements taken during the best viewing conditions.
The annual average when U.C. Davis started its tests in 1968 was more than 100 feet. In 1998 the number was 66 feet; in 1997 it was 64 feet. Besides the past three years, the annual average has not gone below 70 feet.
Richards, who has worked for U.C. Davis since 1969 and has taken many of the measurements, says the change in Tahoe’s transparency is obvious.
“The water is not the deep, cobalt, almost-purple blue as much as it used to be,” he said. “To me, it has more of a cloudy picture than it used to. You used to be able to see much farther in the water.”
The development around Tahoe is largely to blame for the continued loss of clarity in the 23-mile-long, 12-mile-wide, nearly 1,600-foot-deep lake nestled in the northern Sierra. Nitrogen- and phosphorous-rich nutrients from shore erosion, air pollution, untreated stormwater runoff and other factors spur the growth of algae in Tahoe, causing the increased murkiness.
In the 1990s, increased efforts to reverse the declining clarity started happening, and now officials from Tahoe, both Nevada and California, and the federal government are trying to raise money for environmental restoration projects inside the basin.
“Without serious and effective efforts to control nutrient and sediment input into the lake, this declining trend in lake transparency will gradually result in converting Tahoe’s famous cobalt blue waters to green,” said Charles Goldman, the U.C. Davis scientist who started the clarity measurements in the 1960s. “Fortunately, the state and federal agencies working together with the University of California, the University of Nevada and the Desert Research Institute have completed agreements to coordinate their efforts and repair the basin and stem the eutrophication and transparency loss of this uniquely beautiful lake.”
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