Every winter in the high backcountry of the Sierra Nevada, alpine lakes begin to freeze over with a thin layer of ice that slowly grows thicker as the temperature continues to plummet. Over time, snow piles on top, hiding any signs of the lakes until well into spring or even summer.
But the gigantic lake surrounded by these acres of wilderness remains free of the frosty stuff. Why? The short answer: Tahoe is too deep. But the long answer, which takes a look at what is actually happening to the crystal clear water that brings people here in droves, is far more complicated.
“Tahoe doesn’t freeze because there is too much water and too much deep water. Compared to many other lakes, it’s depth-to-area is very large,” confirms Geoff Schladow, director at UC Davis Tahoe Environmental Research Center
The lake is 22 miles long, measured north to south, and 12 miles wide, with 72 miles of shoreline. A maximum depth of 1,645 feet in Crystal Bay makes Tahoe the second deepest lake in the United States, just behind Crater Lake in Oregon, and more than deep enough to submerge the Empire State Building, which measures 1,454 feet at its tip. On average, the lake contains 37 trillion gallons of water — an unimaginably large metric — but that’s enough water to cover a flat area the size of California with 14 inches of water, according to the U.S. Forest Service.
“The surface of the lake is where the lake loses temperature. Winter comes, we start losing temperature out of the surface of the lake through a range of processes, and eventually if it was a shallow lake, say like a reservoir, it would lose enough heat that the surface of the lake would freeze,” explains Schladow. “But at Tahoe, as you keep losing heat, the water is mixing, and the lake never reaches the freezing point before spring arrives and the lake starts heating again.”
The mixing process is key for oxygenating the depths of Tahoe, which unlike most lakes that may have their water replaced once or twice a year, only sees about 0.1% of its water replaced every year, even during an especially wet year. The oxygenation is vital for the survival of the fish and other organisms in the lake.
“During winter in Lake Tahoe, you have heat being lost and we have dense, heavy water, forming at the top because it’s getting colder and colder. That cold water is like an upside down cumulus cloud. It wants to fall down. And as it falls down, it mixes the lake,” says Schladow. “The mixing goes deeper and deeper. 100 feet. 300 feet. 1,000 feet. And some years, it will go all the way to the bottom, and the whole lake is mixed and every part of the lake is losing heat. Most years, that mixing only goes part way down and then spring comes and it stops.”
The cool water mixes with water at the bottom of the lake, which is slowly warming from geothermal heat from the earth.
“One of the things that climate change is doing — and we have this based on 50-60 years of records plus models looking at future climates — is that it’s making the summer periods in Tahoe longer. It means that winter is getting shorter, so that this period when we’re losing heat from the lake is smaller,” explains Schladow. “We’re going to have longer periods of time before we mix all of the way to the bottom, and we’re going to have longer periods of increasing temperature on the bottom before we get that drop again.”
If the lake does not mix to its depth for 20-30 years, it could trigger a release of nutrients from the bottom of the lake, which could have a huge impact on the lake’s famed clarity.
Though the likelihood of Lake Tahoe freezing is nearly zero, one part of the lake does occasionally get a thin layer of ice on its surface: Emerald Bay.
With a depth of about 230 feet and a small channel roughly 6-8 feet deep connecting it to the lake, Emerald Bay is almost like its own body of water, says Schladow. And on some especially cold, windy winters, the surface of the water loses enough heat to freeze. Looking down at the iconic bay, visitors can even see trails through the ice where the M.S. Dixie plowed through as it continued its wintertime tours of Emerald Bay. It’s a sight to behold.