Lake Tahoe is famous for its bright blue water. But according to scientists, the lake’s clarity is under threat.
The UC Davis Tahoe Environmental Research Center publishes a report on the clarity of Lake Tahoe every year.
Their findings for 2019 are out now – and it’s not good news.
The average visible depth for last year declined nearly eight feet from the year before.
Jesse Patterson is chief strategy officer for the League to Save Lake Tahoe. He said the blueness of the water is a reflection of the clarity because the deeper sunlight can penetrate the bluer the water looks.
But the clarity is more than a measure of the lake's incredible blue color, it is a measure of the health of the whole ecosystem, he said.
“So, it is sort of a barometer of the entire lakes’ health. How are we doing at Lake Tahoe? Clarity is one way to look at that,” he said.
Lake Tahoe’s average annual clarity was 62.7 feet in 2019 and in 2017, clarity was worse at 60 feet. At one point, the clarity was 100 feet.
“It is alarming to see two of the lowest numbers in history come in the last three or four years,” Patterson said.
UC Davis has been measuring the clarity of the lake since 1968. Between the late 60s and the early 90s, clarity at the lake was in steady decline, but after very hard work from conservationists and government agencies, the steady decline has ended.
Now, the more troubling problem is the extremes in lake clarity. Patterson said that four or five years ago clarity was measured at 80 feet but now it's back down to just 60.
“It’s really these extremes we’re seeing in the average that is showing us how climate change may be affecting Lake Tahoe,” he said.
Climate change is impacting the lake in several different ways, he said. For one, if there is more rain than snow, more dirt and urban runoff will end up in the lake because it moves faster than snow, which slowly melts over time.
Another issue is lake mixing. In the past, the lake would turn over about every four years, meaning the nutrient-rich cold water at the bottom would come to the top and oxygen-rich warm water would sink to the bottom.
But it takes a cold winter to make the lake flip and there have not been as many cold winters in the area as in the past, which meant the lake flipped just last year -- eight years after its last mixing.
Patterson explained when the nutrient-rich water came to the top that activated algae, which grew rapidly.
“We had a really long period of time last year where the clarity was really bad just because of algae and nutrients and warm water and it affected the entire annual average. So we could have had really good clarity in other parts of the year but that situation…really dropped the average,” he said.
Patterson said researchers expect to see more and more years like that one as the lake mixes less and less.
After years of steady improvements in clarity, researchers are concerned about the future of the lake with continued climate change.
“Climate change just changes the playing field,” Patterson said, “Now, we’re seeing these extremes and we’re like, ‘Wow! Is Tahoe prepared to be resilient to these massive swings in weather and temperatures?’ And can it basically reset itself like it used to every four years? Can it keep itself healthy?”
Patterson said to improve the situation more needs to be done to accelerate the restoration of areas of the Tahoe basin that were clear cut in 1880s, along with repairing meadows and marshes that work as natural filters to runoff.
He said debates over the years among government agencies and the two states that Lake Tahoe straddles have delayed some of that important work.
Jesse Patterson, Chief Strategy Officer, the League to Save Lake Tahoe