Lake Tahoe water clarity improved in 2025, but long-term recovery remains stalled
Lake Tahoe’s water clarity has reached a plateau, showing no significant long-term improvement or decline, even as the annual average has trended downward over the past several decades.
The annual average water clarity measured in 2025 was 69.2 feet — nearly 7 feet better than the average of 62.3 feet recorded in 2024, according to the 2025 Lake Tahoe Clarity Report. by UC Davis’ Tahoe Environmental Research Center. Scientists gauge clarity by lowering a 10-inch white disk, called a Secchi disk, into the lake and measuring the depth at which it remains visible.
Although the 2025 measurement improved compared with 2024, researchers cautioned against celebrating the change, explaining that water clarity remained far below historic levels.
“Water clarity rapidly declined through the late 1990s and then leveled off to what we see today,” Stephanie Hampton, the director of the Tahoe Environmental Research Center, said in a statement. “Statistical analysis shows us that this year was not significantly different from last year, and lake clarity has not substantially improved for decades.”
Declining clarity
UC Davis began tracking the lake’s clarity in 1968, when visibility measured 102.4 feet. Lake clarity has continued to dip ever since. Since 1988, water clarity has remained below 80 feet, with a high of 78.1 feet recorded in 2002 and a low of 60.4 feet measured in 2017.
The states of California and Nevada jointly created the Tahoe Regional Planning Agency, or TRPA, in 1969 in response to early findings indicating the lake’s declining clarity. Along with more than 80 public and private organizations, the TRPA is working to restore lake clarity to 97.4 feet — the average level recorded between 1967 and 1971 — under the Lake Tahoe Environmental Improvement Program, according to Kat Kerlin, a spokesperson for the UC Davis Tahoe Environmental Research Center.
Causes of cloudy waters
Researchers have identified fine particles in the lake’s upper waters as one of the key factors affecting water clarity. Fine particles include microscopic sediment, debris and organic matter, including phytoplankton, wildfire ash, decaying plant debris and clay. They enter the lake from streams and the atmosphere or occur naturally in the lake. Seasonal peaks in fine particles in 2025 correlated with water clarity, aligning with findings from recent years.
However, for scientists seeking to restore the lake’s clarity to its historic levels, the correlation does not explain why clarity has plateaued — the answer appears to lie in particle research, according to Hampton.
“Over the past several decades, water clarity has plateaued rather than improved,” Hampton said. “Why isn’t clarity improving more? To really answer that question and find out what is currently driving water clarity, examining the nature of the particles is a key research priority.”
Researchers will employ new techniques in 2026 to investigate the lake’s phytoplankton community — including its smallest component, picoplankton — to determine the extent to which phytoplankton and picoplankton influence clarity. Using holographic imaging technology, scientists will also investigate particle aggregation to determine how the lake’s physical dynamics cause particles to clump together and sink, affecting water clarity.
“These approaches aim to combine new science and engineering perspectives to understand the interconnected system of Lake Tahoe and help shape the future of clarity research at the lake,” Kerlin said in a statement.
