Sunflower Flat Hot Spring
Sunflower Flat Hot Spring surfaces at 109 degrees Fahrenheit in the Challis National Forest, hitting that narrow temperature window where a natural soak feels just right without needing river water to cool it down. The alkaline water, with a pH of 9.2, carries elevated sulfate at 110 mg/L.
The approach covers roughly 1,393 meters through open forest and sagebrush flats at 4,480 feet elevation in Custer County. The terrain here is drier than Idaho's western mountains, receiving just 22 inches of precipitation annually. The spring emerges in a flat clearing where mineral deposits stain the surrounding rocks in muted orange and white. Ponderosa pine and Douglas fir provide partial shade. The sulfate-rich water leaves a faint mineral smell that is noticeable but not overpowering.
The short walk of about 0.9 miles is manageable for most hikers in dry conditions. No formal trail exists, so use GPS coordinates for navigation. The nearest reservable campground, Boundary Creek, sits 31 kilometers away. Bring all supplies including water. The 109-degree temperature is comfortable for most people without requiring cold water mixing, but test before entering. Snow limits access roughly November through May.
Wikipedia identifies Sunflower Hot Springs as a geothermal mineral spring in the Boise National Forest, though the administrative boundary places this particular feature within the Challis National Forest. USGS measurements confirmed temperatures around 47 degrees Celsius. The water's high sodium content at 97 mg/L and extremely low magnesium at 0.1 mg/L indicate deep circulation through silicic volcanic rock rather than contact with sedimentary formations.
The water at Sunflower Flat Hot Spring is alkaline (pH 9.2).
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Overview Boundary Creek Campground is located at the edge of the Frank Church River of No Return Wilderness, adjacent to the boat launch for the Middle Fork of the Salmon River in Idaho. The site generally provides overnight camping for boaters waiting to launch the next day on their permit to float the Middle Fork. Of the 15 campsites, 5 can be reserved during the high-use season (June 15-Aug. 15); the remaining 10 are first-come, first-served. Outside of those dates, all 15 sites are first-...
Sunflower Flat Hot Spring is road-accessible, so you can bring more gear.
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