Mormon Ranch Warm Spring
Mormon Ranch Warm Spring requires a three-mile hike into Idaho's Salmon National Forest, rewarding backcountry travelers with 117°F waters at 4,300 feet elevation in the remote Yellowjacket drainage of Custer County. The alkaline water (pH 9.4) runs low in minerals, suggesting deep geothermal circulation through granite.
The trail winds through rugged mountain terrain where the Yellowjacket drainage carves its way through Custer County's backcountry. At 4,300 feet, the forest canopy thins enough to reveal ridgelines in every direction. The spring emerges into a landscape that receives over seven and a half feet of snow annually, meaning summer access comes with wildflower meadows fed by snowmelt, while winter buries the approach under deep drifts. Silence here is near-total, broken only by wind and water.
Plan for a full-day outing or overnight trip given the three-mile hike from the nearest road. The historic Yellowjacket Guard Station, an original 1925 Forest Service building remodeled by the CCC in 1934, is reservable and sits about 11 miles away. Snow typically blocks access from late fall through spring. Carry all water and supplies.
The spring rises from fractured rock deep in the Idaho Batholith, a massive body of Cretaceous granite that underlies much of central Idaho. Its strongly alkaline pH of 9.4, low calcium (2.8 mg/L), and elevated sodium (50 mg/L) indicate water that has circulated deep enough to dissolve silicate minerals over long residence times. This chemistry is typical of the region's fault-controlled thermal systems.
The water at Mormon Ranch Warm Spring is alkaline (pH 9.4).
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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...
Mormon Ranch Warm Spring is road-accessible, so you can bring more gear.
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