Cox Hot Springs
Cox Hot Springs requires a 2.7-mile hike into Boise National Forest to reach, making it one of the more committed outings among Custer County's thermal features. The 131-degree Fahrenheit water at 4,160 feet elevation carries alkaline pH of 8.9 and 100 mg/L sodium with minimal chloride, marking clean volcanic-type geothermal chemistry.
The trail crosses forested backcountry in the remote reaches between Challis and the Middle Fork drainage. At this elevation, ponderosa pine gives way to lodgepole and subalpine fir on north-facing slopes, and the terrain is broken by small meadows and rocky outcrops. Annual snowfall tops seven and a half feet, and the area is effectively snowed in for five to six months. The spring's thermal output is visible as steam threading through the tree canopy on cold days. Solitude is the default here.
Cox draws from the same Idaho Batholith geothermal system that feeds dozens of springs across central Idaho. Its 8.9 pH, 100 mg/L sodium, 47 mg/L sulfate, and low chloride at 12 mg/L indicate deep circulation through granitic rock with extensive silicate mineral dissolution. The near-absence of calcium at 1.8 mg/L confirms thorough ion exchange at depth. Recent USGS measurements put the temperature at 39 degrees Celsius, cooler than the historical 131-degree Fahrenheit record.
Yellowjacket Guard Station, a reservable historic 1925 Forest Service structure, sits about 37 miles away. The hike in is roughly 2.7 miles with no maintained trail infrastructure at the spring itself. Bring a detailed topographic map or GPS, as route-finding is part of the challenge. The discrepancy between historical and recent temperature readings means water conditions may vary. Best visited July through September when snow has cleared.
Is Cox Hot Springs worth visiting?
Best for
- Hot spring soaking
- Overnight camping trips
The water at Cox Hot Springs is alkaline (pH 8.9).
Overview Yellowjacket Guard Station is a historic Forest Service Ranger Station and Work Center. Originally constructed in 1925 and remodeled in 1934 by the Civilian Conservation Corps, the structure is eligible for listing on the National Register of Historic Places. Recreation Hikers, horseback riders, mountain bikers and off-road enthusiasts will find many trails in the surrounding area. Hunting is also a popular activity. Click here for an Idaho OHV safety messageFacilities The spacious...