Lake Tahoe is huge and sublime. Mark Twain called it the “fairest picture the whole Earth affords” after his visit. Straddling the California-Nevada border, the “vast oval” of blue described by Twain measures 35 kilometres long by 19 kilometres wide. It’s also uncommonly deep, a maximum 501 metres, making it the 10th deepest lake in the world.
That depth, along with the lake’s steep sides, give it enormous holding capacity, 39 trillion gallons. It’s enough capacity that the water slopping over the rim into the Truckee River and flowing past Squaw Creek, Northstar and other ski areas, entered the lake some 600 years ago, about the time that a young Joan of Arc was growing up on a French farm.
Geoffrey Schladow used a different metric for describing the lake’s vast quantity. It’s enough, he said, to cover the entire state of California in 38 centimetres of water. “It’s a very steep-sided lake and it’s very deep,” said Schladow, the director of the Tahoe Environmental Research Center. “I have done that calculation several times to convince myself of it.” That’s a lot of water to warm, and it takes a great deal of energy to do so, Schladow explained.
But the lake is warming. The latest State of the Lake report issued by the research centre in late July concluded that the average water temperature in the lake increased nearly a quarter degree Celsius in just one year, 2014-2015. It was a drought year and an uncommonly warm year altogether. Still, that’s 15 times the long-term rate of warming.
Warming has taken centre stage at Lake Tahoe as scientists continue to monitor the ecology of the lake and its surroundings. For many years, clarity was the central focus, and it remains a concern. Warming and lake clarity are bound together in mountain lakes. But Tahoe’s original problem had a different origin.
Some think that it was once possible, such as when Mark Twain first laid eyes on what he called the “noble sheet of blue water” in 1861, to see to a depth of 37 metres. When measurements began in 1968, scientists could see a white Frisbee-type instrument, called the Secchi disk, to a depth of 30 metres. After that, clarity receded to 21 metres in the late 1990s.
Scientists concluded that the problem was how people were using the land along the shores of the giant lake. It was no one thing, but the complex of roads, streets, and houses resulted in more deposition of nitrogen and phosphorous into the lake. It’s called “cultural eutrophication,” defined by an advocacy group called Keep Tahoe Blue as “excessive algal growth due to excessive nutrient levels.”
More important yet are the fine sediments — tiny, ground-up particles, smaller than the width of a human hair — that have been entering the lake. Rather than falling to the bottom, they remain suspended, dulling the transparency of the water.
That transparency has actually been improving. A summit in 1997 drew then-U.S. President Bill Clinton and other high-ranking officials from California and Nevada and produced a US$50 million federal commitment. There has been success.
Clarity had been receding a third of a metre a year, but in the last 20 years has started improving. The Seechi disk could be observed to a depth of 24 metres in 2014.
On Aug. 31, President Barack Obama will visit the lake to address the annual Lake Tahoe Summit. A White House spokesman said Obama would use the opportunity to underscore a commitment to addressing climate change and preserving the country’s natural treasures for future generations.
This year’s State of the Lake report focuses on that changing climate in the Tahoe Basin. There’s a lot to talk about. One hot topic is the shift from snow to rain. In 1910, snow was responsible for an average 51 per cent of total precipitation. In recent decades, that’s dropped to 33 per cent. But in the water year of October 2014-September 2015, just 6.5 per cent of precipitation fell as snow. Total precipitation that year was about two-thirds of the average. Last winter — the stuff for next year’s report — was closer to normal in terms of snowfall.
The larger story is rising temperatures. The shift is most pronounced in nightly minimums. Since 1911, the average daily minimum temperature has increased by 2.4 degrees C. The average daily maximum temperature has gone up 1.1 degree C.
The warming water is likely a result of warming air temperatures, Schladow said.
Warming water has implications for mixing of water and hence for clarity. Precipitation arriving as rain, instead of snow, is inherently warmer, and as it enters the lake it tends to stay higher in the lake, introducing sediments that deteriorate lake clarity.
Other lakes are also changing as a result of warming temperatures. Schladow was in Italy last week at a conference of scientists devoted to the issues of reduced lake mixing. Many lakes in Europe have much greater reduced mixing than what is being observed at Lake Tahoe. The reduced mixing eliminates oxygen from the lake bottoms, making them uninhabitable by fish.
“You’re seeing a lot of undesirable chemical changes,” said Schladow. “We are a long way from that situation in Lake Tahoe, but if don’t have the usual amount of mixing, we will move to that in the future.”
He sees warming creating new dynamics for Tahoe and other lakes. “What I think is important about climate change is not just that the water is getting warmer, but that it’s starting to change the ways in which lakes work,” he said.
“The fact that some of the water on top is warmer in a cold lake makes water move in particular ways. We talk about how oxygen gets to the bottom of the lake. Climate change is altering that. It’s not just changing temperatures, it’s changing how lakes operate.”
At Lake Tahoe, he said, broad global impacts playing out locally are starting to have a greater impact than urbanization in the basin. “We are getting to the point that climate change is maybe approaching the magnitude of factors like urbanization around the lake,” he said before citing a litany of the changes.
But as other scientists involved with water have started to point out, Schladow wondered if it’s time to reassess standards for action. Many standards were developed in the American West during the 20th century, based on historical records. Climate change is creating new normals — and the normals are constantly changing.