�������

The annual “” report from the Tahoe Environmental Research Center at the �����о���, released today (Aug. 8), shows how long-term climate trends continue to affect the iconic lake in complex ways. The report also describes how research at Lake Tahoe relates to work at other lakes in the U.S. and worldwide, including the development of new technology, and how citizens are contributing to preserving Tahoe’s natural beauty. 

“From wildfires to weather extremes, climate change is impacting Lake Tahoe just as it is lakes and rivers across the country. Our long-term research at Lake Tahoe can help us better understand and perhaps mitigate these impacts,” said Alexander Forrest, interim director of the �����о��� Tahoe Environmental Research Center and professor in the Department of Civil and Environmental Engineering.

The State of the Lake report is intended as an annual update for non-scientists on conditions at the lake, framed in a historical context. �����о��� researchers have been monitoring conditions at Lake Tahoe continuously since 1968. 

Weather and climate

Long-term meteorological trends at the lake are for generally rising air and water temperatures, with more precipitation falling as rain rather than snow. 2023 showed some variance from this trend: Average surface water temperatures were in line with the rising trend, but air temperatures were colder than the trend, there were more days with freezing temperatures, and more precipitation fell as snow. 

These colder conditions in winter 2023 enabled a “deep mixing” event to take place, exchanging water from the bottom to the top of the lake. This allowed the lake to lose some heat – contributing to a lower average temperature for the entire lake – and also provided some of the best wintertime clarity on record, with a measured value of 91.9 feet. 

However, sediment being transported from that meltwater from the large winter snowfall, along with the growth of microscopic plants and algae, contributed to a decline in clarity during the summer to 53.5 feet, the fifth-worst summer figure on record. 

While winter 2023 at Lake Tahoe was colder than the recent trend, extreme weather events such as powerful winter storms and heavy snowfall are a consequence of climate change. 

At the end of 2022, the lake was just below its natural rim, which is at 6,223 feet above sea level. The lake level rose steadily through 2023, reaching a maximum of 6,228 feet in July before water releases and evaporation from the lake reduced the volume of the lake. 

There have been several episodes over the past 114 years where the lake level fell below the natural rim, meaning that water no longer flows out through the Truckee River. 

Biology

The deep mixing event in early March and fine particles swept into the lake by runoff events in May and June affected the amount and distribution of nitrogen and phosphorus in the lake, both important nutrients for plankton. These tiny plants (phytoplankton) and animals (zooplankton) are the basis for the food web in the lake. 

“Over the years we’re seeing changes in the phytoplankton and zooplankton communities, but we don’t fully understand yet what the implications are for clarity,” Forrest said. 

The dominant phytoplankton species, Cyclotella, has declined since 2017 while another, Synedra has expanded. Populations of non-native Mysis shrimp are making a comeback after collapsing in 2022, but native copepods are also growing in number. 

“We need to understand these long-term changes to better predict the state of the lake in the future,” Forrest said. 

Technology testbed

Tahoe is both unique and one of thousands of mountain lakes across the American West. It can be both an example and a testbed for environmental science and technology. 

TERC scientists Adrianne Smits and Professor Steve Sadro are studying other lakes in the Tahoe region as part of the California Mountain Lake Network, a set of 15 lakes representing different elevations and environments. Their goal is to understand the extent to which California’s alpine lakes are sensitive to environmental change, and to develop an understanding that informs lake management. They contributed to research published earlier this year showing that between 2019 and 2021, almost every lake in North America was affected by wildfire smoke on at least one day a year. 

TERC has worked with the Korean Polar Research Institute to develop underwater ‘gliders’ for use under the Antarctic ice shelf, and with Swiss scientists on similar technology to study ice-dammed lakes in the high Arctic. 

Human impacts

In the historical record of Lake Tahoe’s clarity, there is a clear improvement in the early 2000s, as efforts to protect and restore the lake came into effect. The continuing improvements in winter water quality show that these efforts are having a long-term effect. 

At the same time, the Tahoe Basin is not an isolated system. Wildfires as far away as Canada deposit smoke particles into the lake. Droughts in California leave Tahoe dry, as well. 

An important role for TERC is conducting public education about Lake Tahoe, its ecosystem and environment, and how they are affected by regional and global trends. The goal is to provide science-based information about the Lake Tahoe region, to foster responsible action and stewardship. In 2023, the TERC education team reached almost 13,000 visitors through tours, field trips, lectures and community events.

TERC researchers work with other scientists, notably at the University of Nevada at Reno and the Desert Research Institute, in part through the Tahoe Science Advisory Council. Established in 2015 by the states of California and Nevada, the council is an independent group of scientists who work collaboratively to advise policymakers to promote, enhance and maintain the ecological integrity of Lake Tahoe and its watershed. 

Visitors and residents can contribute to protecting the lake as volunteer citizen scientists, reporting observations on algae, invasive species, litter, water quality and ashfall from wildfires, among other things.