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…