Oregon’s Crater Lake is, essentially, a giant, clear-blue puddle, 1,984 feet deep. It’s the deepest lake in America, and the seventh deepest in the world. If you plopped One World Trade Center down on the lake’s lowest point, its tip would still be under water. The lake’s basin crumbled into being 7,700 years ago, when volcanic Mount Mazama collapsed into its empty magma chamber, creating a caldera, a bowl-shaped indent on top of the mountain.
However, its incredible depth is perhaps less important than its clarity. According to the National Park Service, Crater Lake might be the clearest lake in the world—“waters of Crater Lake are actually resetting the standards for the optical properties of ‘pure water’.” Every other winter or so, the lake undergoes its version of a cleanse. In these “deep ventilation” events, unseen movements of the water carry oxygen downward and bring up nutrients. Deep ventilation feeds what little life there is the lake and move the nutrients and the algae around in ways that spur or inhibit growth. Naturally, because this is how the world works these days, climate change will modify this process.
This is all to say, Crater Lake’s future is not clear.
In a study the U.S. Geological Survey released in May, scientists found that as global average temperatures go up, deep ventilation will happen less often at Crater Lake—anywhere from once in three years to once in 7.7 years, depending on the speed and degree that temperatures rise.
Most lakes, fed by rivers and streams, are full of life and particles. Crater Lake, fed only by snow and rain falling into the caldera, has very little in it other than water. That’s why it’s so clear. At times, it’s been possible to see 130 feet down into its depths.
“It’s got this teeny little basin that’s defined by the rim of that crater,” says Tamara Wood, the lead author of the new study and a hydrologist at USGS. There’s very little area from which nutrients can run off into the lake—it’s almost a closed system. “There’s a lot of just moving things around,” she says.
Deep ventilation, a very important part of moving things around, only happens under special circumstances, in which pressure, density and temperature conspire to move water down deeper into the lake than it would normally go. At most times of year, the lake is warmer on the surface, but colder on the bottom. (You understand this if you’ve swum in a summer lake—it’ll be pleasant until you let your toes drop deep, where the water is frigid.)
The cold water at the bottom of the lake stays put, because water is denser when it’s colder—up to 4 degrees Celsius. That’s the point where water is heaviest; at colder temperatures, it starts becoming less dense again. (Like how ice floats in part because it’s less dense than water.) In the winter, when temperatures drop low enough, less dense, very cold water will make up the lake’s top layer, and it’ll be warmer below.
But pressure changes that equation. In a lake as deep as Crater Lake, there can be a significant change from atmospheric pressure down deep in the lake, and as the pressure increases, the temperature at which water is heaviest drops. When the wind pushes cold surface water down to that warmer, higher pressure layer, all of a sudden it can’t float anymore. The water from the surface is still cold, but, under more pressure, temperature translates to greater weight.
The water starts to sink, and it keeps falling, like Alice down the rabbit hole, until it reaches a place where it’s as dense as the surrounding water or the bottom of the lake, whichever comes first. There is a small food web in the lake, and this movement brings oxygen down to the microbes living deep down in the water and helps bring nutrients that have settled to bottom of the lake back up, feeding algae on the top.
The correct conditions for deep ventilation don’t always come together. Right now, the lake flip-flops its water in this way once every two winters. Wood and her colleagues modeled the process going out to 2100, with the climate changing, and they found that the right alchemy will happen less often in a warmer world.
What does that mean? In the paper, Wood and her colleagues are careful to say that they can’t predict how this will affect the growth of algae in the lake or the lake’s water clarity. They modeled only the physical changes that might happen, not biological ones. Without the nutrients that mixing provides, the algae on the surface could die, making the lake more clear; warmer temperatures could also affect the ecology of the lake by allowing a longer growing season, though, increasing the amount of algae. When the mixing does happen, it could bring up a feast of nutrients settled on the bottom and feed a population boom in algae, which could create green blooms of life on the lake’s surface.
“These linkage between the climate and the freshwater system, they’re intricate and complicated,” says Wood. “It’s not obvious how this is all going to play out 600 meters below the surface.”
At Lake Tahoe, for example, another deep and very blue lake, climate change is also a concern, and the water at the surface has been less clear in years when deep mixing hasn’t happened. Tahoe’s clarity started dropping a few decades ago, when fine, inorganic particles started accumulating in the lake. (Once they would have been trapped in the surrounding land but after the area had been more heavily developed, those particles began washing off roads and through drains, into the lake.) Deep ventilation helps because when the less particle-y water comes up from the bottom, it improves clarity at the surface.
If people can keep from adding extra organic matter and particles to the lake, though, the lake will be more resilient to climate changes. “If there’s less to stimulate algal growth, the longer the lake can withstand periods of no mixing,” says Geoff Schladow, director of the Tahoe Environmental Research Center.
Less frequent ventilation could still dramatically alter circumstances for fish, algae and other tiny lake lovers. For humans, though, any change to the lakes’ looks would be subtle. “I don’t want you to think Lake Tahoe is doomed,” says Schladow. “It’s not—it’s beautiful out.” A visitor to Crater Lake decades in the future might see an algal bloom, but they might not. If there is a change to the lake’s clarity, it’ll be hard for a visitor to tell, says Wood. Just as it’s not always obvious when a person’s insides are blocked up, the lakes might still look good on the surface, even if they’re suffering inside.