Reno Gazette Journal: Declines in Lake Tahoe's clarity have been halted; scientists ponder what's next

Declines in Lake Tahoe's clarity have been halted; scientists ponder what's next
Amy Alonzo
July 16, 2022

Declines in Lake Tahoe’s overall clarity have largely plateaued, according to recent measurements.

And that’s a win, for some Tahoe scientists.

“I’m cautiously optimistic,” said Alan Heyvaert, associate research professor at Reno’s Desert Research Institute. “The fact that we arrested the decline in clarity – that’s amazing that we’ve done that.”
But work to restore the lake’s clarity to levels not seen in more than 50 years is moving slowly. Measurements show two diverging trends: Summer clarity continues to decline at just under 7.5 inches per year, while winter clarity is generally holding steady. Last year, average clarity in the lake measured just 61 feet, according studies by the University of California, Davis Tahoe Environmental Research Center and the Tahoe Science Advisory Council. The number marks a drastic decrease in clarity from when levels first were measured in 1968, when average clarity was 102.4 feet. It has Tahoe scientists scratching their heads at what the next steps are to reverse decades of declining clarity.

More development, more runoff
Fine particles and tiny algae are responsible for up to 70 percent of the loss of clarity in Tahoe. The particles come from a variety of sources including road runoff, atmospheric dust and wildfire smoke deposits.
“They (fine particles) are so small they don’t actually sink very fast. They can stay suspended in the water column for a long time, and that’s what causes the decline in clarity,” according to Joanna Blaszczak, assistant professor at the University of Nevada, Reno.
The decline in Tahoe’s clarity dates back to a 1960s building boom in the basin. In 1960, the first-ever-televised winter Olympics were held at Palisades Tahoe – then known as Squaw Valley – on the north shore of the lake. That same year, developers on the east shore subdivided a parcel in the new town of Incline Village into 1,700 lots. By 1968, more than 3,000 houses had been built in Incline Village. In 1961, Homewood Ski Area on the west shore was developed. Heavenly Valley, which opened in 1955, expanded its resort into Nevada in 1968.

And for years, Tahoe’s wastewater was channeled into the lake. The mantra during the 1950s and '60s was “dilution is the solution to pollution,” according to Heyvaert. But people started to notice impacts on the lake, and by the mid-1970s, wastewater treatment facilities had been constructed in the basin. “Without that, we wouldn’t even be talking about restoring the lake at this point," Heyvaert said. "That prevented a catastrophe.” As steps were taken to limit wastewater impacts on Tahoe, population continued to grow around the lake. Between 1960 and 1980, Tahoe’s population grew from 10,000 people to 50,000. Now, in addition to a growing year-round Tahoe community, about 15 million people visit the lake annually. The increases in development and population led to more urban pollution and runoff. It became clear by the 1990s that more concerted efforts were needed, Heyvaert said.

A long road to recovery
In 2011, a plan requiring local governments and highway departments to limit clarity-harming pollutants that wash into the lake was implemented. The goal: Restore Tahoe’s clarity to 97.4 feet, a level that hasn’t been seen since 1970.
But the lake is slow to change, Hayvaert said. It hasn’t recovered from a deluge of fine particles that flowed into it during heavy storms and floods in 2017. That year marked the worst clarity ever recorded at Tahoe. The average annual clarity level in 2017 was 59.7 feet, a 9.5-foot decrease from 2016. Prior to that, the worst clarity was measured in 1997, also a flood year, when visibility was only 64.1 feet. Fine particles in the streams that feed Tahoe increased four-fold during in 2017 and have remained elevated since then, despite less water flowing into Tahoe with drought conditions. “It’s like a battleship instead of speedboat. It doesn’t change direction quickly,” Heyvaert said of Lake Tahoe. The elevation in fine particles despite less runoff is counter to what scientists think should be happening, according to University of Nevada, Reno associate professor Sudeep Chandra. With less water flowing into the lake, less particles should be transported into it, leading to some improvements in clarity. Despite the mystery of why summer clarity is still decreasing, Heyvaert thinks the glass is half full.

“The jury is still out on whether we are on pace to hit the clarity target in 2076, but there is progress in simply having arrested the long-term decline in lake clarity at this point,” he said. “Now we have a more difficult job of ultimately restoring the clarity to historic conditions, which means reversing that trend.”

Amy Alonzo covers the outdoors, recreation and environment for Nevada and Lake Tahoe. Reach her at aalonzo@gannett.com. Here's how you can support ongoing coverage and local journalism.

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