UC Davis: Lake Tahoe surface warming is strengthening summer stratification and posing clarity risks

Dr. Stephanie Hampton, director of the UC Davis Tahoe Environmental Research Center, told the Placer County Board of Supervisors on Sept. 22 that Lake Tahoe’s surface waters have warmed over recent decades, strengthening summer stratification and lengthening the season during which surface and deep waters do not mix — changes that reduce oxygen transfer and raise questions about long‑term clarity.

Hampton said long‑term records show an annual average lake warming rate of 0.21°F per decade and a surface warming rate of about 0.39°F per decade, with the largest warming occurring in summer. She told the board that the lake’s seasonal stratification is now 13.8% stronger than in the past and the stratified ("summer") season is about 29 days longer over the 56‑year record the center analyzed.

The longer, stronger stratification matters because "warm water floats on top of cooler water," Hampton said, explaining that reduced mixing limits oxygen delivery to deeper waters and changes the movement of sediment, nutrients and pollutants. She said the region has also warmed overall: the number of days with average air temperature below freezing has declined by more than 27 days since records began in 1911, and more precipitation now falls as rain than snow (about 30–40% as snow today versus roughly half around 1910).

Why it matters: clarity, the metric most familiar to many Tahoe residents, is influenced by fine particles and algae. Hampton described a detailed UC Davis study from about 20 years ago that found clarity‑reducing particles were roughly 58% inorganic (fine sediment) and about 25% organic (algae). She said phosphorus — a nutrient that can support algal growth — appears to be rising again in recent years, though the cause is not yet clear.

Hampton summarized monitoring and planned research. UC Davis operates 10 nearshore stations and four mid‑lake buoys that provide near‑real‑time data; one of the ways the center measures clarity is the traditional Secchi disk. The center is testing expanded nearshore monitoring that combines diver observations and water sampling with aerial imagery (drones and helicopter perimeter flights), machine‑learning image analysis and a graduate student trialing satellite remote sensing. The team is also beginning eDNA sampling to detect aquatic invasive species and is examining the relative roles of sediment, ash (from wildfires) and microplastics in reducing clarity.

On monitoring specifics, Hampton told the board that UV penetration and turbidity profiles are measured through the water column; a study the center published last year showed higher UV penetration in drought years when fewer watershed materials enter the lake. Hampton said standard clarity sampling is performed at least monthly and many nearshore measures are collected about every 10 days.

Board questions and follow‑up: Supervisor DeMattei asked about the depth of the warmed surface layer compared with 50 years ago; Hampton said she did not have that specific depth off the top of her head but that the data exist and she would provide an answer at a future meeting: "I don't know the answer off the top of my head, but I know that we have the data to answer that because it doesn't correspond directly to the, how much warming occurs in the surface water. ... I'm gonna have the answer next time we get together." Supervisor Jones asked about how current data extend through 2024; Hampton said most datasets extend to 2024 and the report cut-off was 2024 so a full last‑year report could be prepared by March following the sampling year.

Research priorities Hampton identified include: (1) determining whether clarity‑reducing particles are still predominantly fine sediment or whether algae, ash or microplastics are increasing in importance; (2) better understanding when stream inputs plunge into deep water versus float and affect surface clarity; and (3) scaling nearshore monitoring with aerial imagery and automated analysis. She noted the center’s public‑facing work — outreach exhibits and online real‑time data — and said more than 10,000 residents and visitors engaged with TERC exhibits and resources in 2024.

Hampton closed by stressing the need to maintain long‑term observational infrastructure and partnerships across UC Davis, Desert Research Institute, USGS, the Forest Service and local agencies to translate science into local management.

The meeting did not include formal board action on the report; Hampton volunteered to return with specific follow‑up data on requested points.