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.
History of Sunflower Flat Hot Spring
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 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.
Is Sunflower Flat Hot Spring worth visiting?
Best for
- Hot spring soaking
- Overnight camping trips
Sunflower Flat Hot Spring is well suited for hot spring soaking and overnight camping trips.
Annual Precip: 22.0 in
Annual Snow: 98.3 in
December
Air 23°F · Water 109°F · +86° contrast
January
Air 26°F · Water 109°F · +83° contrast
February
Air 29°F · Water 109°F · +80° contrast
December offers 48°F more contrast than July
Recent Temperature
116.6°F
(47.0°C)
Measured Jun 27, 1980
The water at Sunflower Flat Hot Spring is alkaline (pH 9.2).
pH influences mineral absorption through skin
May support bone density and joint health
Known for muscle relaxation and stress relief
Supports circulation and skin hydration
Traditionally used for skin conditions and inflammation
Natural antiseptic with skin health benefits
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-...
Reserve on Recreation.gov →
Swimsuit & towel
Water bottle
Stay hydrated, especially in hot water
Camping gear
Nearby camping at Boundary Creek Campground
Trash bag
Pack out what you pack in
34 miles from Challis, ID
USGS Quad: (GREYHOUND RIDGE 15) · NOAA: 8.0 · AMS Map: CHALLIS
What is the water temperature at Sunflower Flat Hot Spring?
Sunflower Flat Hot Spring has a water temperature of 109°F (43°C). This is considered a hot spring, well above body temperature.
Where is Sunflower Flat Hot Spring located?
Sunflower Flat Hot Spring is located in Custer County, Idaho at 4,480 feet elevation, near Challis. Coordinates: 44.7330°N, 115.0180°W.
How do I get to Sunflower Flat Hot Spring?
Sunflower Flat Hot Spring has Short Walk road access (0.9 mi from the road).
Is Sunflower Flat Hot Spring free to visit?
Sunflower Flat Hot Spring has Open access. It is managed by US Forest Service.
Can I camp near Sunflower Flat Hot Spring?
Yes, Boundary Creek Campground is located nearby (19.4 miles away).
What minerals are in the water at Sunflower Flat Hot Spring?
Water chemistry analysis shows the presence of pH, Calcium, Magnesium, Sodium, Sulfate, Chloride. The pH is 9.2 (alkaline).
Is it safe to soak at Sunflower Flat Hot Spring?
Sunflower Flat Hot Spring is generally safe for soaking at its recorded temperature. Always tell someone your plans before visiting remote hot springs.
Is Sunflower Flat Hot Spring crowded?
Sunflower Flat Hot Spring sees varying levels of visitors depending on the season and day of the week.
What should I bring to Sunflower Flat Hot Spring?
Essentials for Sunflower Flat Hot Spring: swimsuit, towel, water bottle, camping gear, a trash bag to pack out what you pack in.