Hot springs in California

California's thermal springs cluster where active faults and young volcanic centers meet groundwater. East of the Sierra Nevada, the range-front faults and the Long Valley Caldera, a volcanic depression the U.S. Geological Survey dates to a large eruption about 760,000 years ago, heat the springs along U.S. 395 in Mono County. The USGS California Volcano Observatory monitors the caldera. Other groups lie in the desert basins of the Basin and Range in Inyo and San Bernardino counties, around the Lassen and Mount Shasta volcanic areas in the north, and along faults in the Coast Ranges.

On the east side of the Sierra, the Inyo National Forest manages the Hot Creek Geological Site, where a Forest Service order bars entry onto national forest land within 10 feet of the creek because of unpredictable thermal activity; the current order runs through December 31, 2026. The Bureau of Land Management's Bishop Field Office manages much of the open land in the Mono Basin and the Owens Valley. The springs of Saline Valley lie inside the largest national park in the contiguous United States, managed by the National Park Service, and the Sequoia and San Bernardino national forests manage springs along the Kern River and in the desert foothills of Southern California.

Seasonal patterns run in two directions. Caltrans closes State Route 120 over Tioga Pass, State Route 108 over Sonora Pass and State Route 89 over Monitor Pass in winter, and the Tioga closure follows Yosemite National Park's winter closure of Tioga Road. State Route 203 beyond Mammoth Lakes typically closes from early November into late May or June, and chain controls are posted on U.S. 395 during storms. In the low deserts the limit is summer heat: the National Park Service lists the park's hottest recorded air temperature as 134 degrees Fahrenheit, measured at Furnace Creek on July 10, 1913.