Hot springs in Idaho

Idaho holds one of the densest groups of thermal springs in the West, and most of them rise from the granite of the Idaho batholith in the central mountains. A U.S. Geological Survey study of the batholith found that springs issuing from granitic rock sit mostly along major regional faults, and that their dilute water points to deep circulation of rain and snowmelt. The same study measured spring temperatures from 20.5 to 94.0 degrees Celsius. Later USGS dating work estimated that the water in these systems fell as precipitation between about 10,000 and 33,000 years ago. To the south, the Snake River Plain marks the track of the Yellowstone hotspot across the state.

The great majority of these springs lie on national forest land: the Boise, Payette, Sawtooth and Salmon-Challis national forests in the batholith country, and the Nez Perce-Clearwater National Forests along U.S. 12 and the Lochsa River. Rules vary by site. At Jerry Johnson Hot Springs, reached from the Warm Springs Trailhead on U.S. 12, the Forest Service allows day use only, from 6 a.m. to 8 p.m., with no overnight camping. The Bureau of Land Management manages most of the public land in the desert of southern Idaho.

Winter road closures are common in the central mountains. The Idaho Transportation Department closes State Highway 21 between Grandjean and Banner Summit, an 11-mile stretch with about sixty regular avalanche paths, several times each winter, and stopping on that section is prohibited in winter. When it is closed, the posted alternate route runs east on Interstate 84 to Mountain Home, then on U.S. 20 and State Highway 75. Many forest roads to backcountry springs are not plowed.