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LAKE TAHOE, Calif. (KRON) — Mark Twain once described Lake Tahoe as “a noble sheet of blue water. It must surely be the fairest picture the whole earth affords.”

The lake is blessed with beauty thanks to its iconic cobalt blue waters. How is this natural jewel in Sierra Nevada mountains so phenomenally blue?

Scientists at the University of California Davis measured a “quantification of Tahoe’s blueness” based on wavelengths of light coming from the lake. Data from a NASA-operated buoy in the lake enabled researcher Shohei Watanabe to create a Blueness Index that quantified Lake Tahoe’s color for the first time.

“What the research is revealing is that while clarity is controlled by fine particulates (sediment), blueness is controlled by algal concentration,” UC Davis researchers wrote. “The lower the algal concentration, the bluer the lake.”

Algal concentration is controlled by the level of nutrients available to the algae.

With depths of 1,645 feet, Tahoe is North America’s second-deepest lake. What’s at the bottom (and what isn’t) also contributes to the lake’s color.

“One of the most interesting things to find at the bottom of the Lake is, essentially, nothing. While there are some interesting old relics — sunken boats and trash from decades ago — the lake is largely a desert. And that is precisely why it’s so clear, clean and has that famous, piercing, deep blue color,” said Chris Joseph, communications director for League to Save Lake Tahoe.  

“If it were more rich in nutrients, plant and animal life, it’d look more like the cloudy, greenish lakes across the country and world. So it’s really Tahoe’s lack of ‘stuff’ that makes it so attractive,” Joseph said.

Lake Tahoe is iconic not only for its blueness, but also its clarity. That’s where nonprofit groups like League to Save Lake Tahoe and Clean Up The Lake play a pivotal role.

Volunteer teams of beach combers and scuba divers collect thousands of pounds of trash from the lake every year. Clean Up The Lake said some unique trash items it found in the lake were vinyl records, a 9-mm handgun wrapped in a black plastic bag, bones under a shipwreck, and religious statues.

To keep Tahoe blue, the alpine lake must remain free from nutrient runoff that feeds algae. Trash and sediments must also be kept in check.

“That’s why Keep Tahoe Blue works on keeping stuff (trash, runoff, invasive species) out of the lake,” Joseph said.