Foster Ranch Hot Springs
Foster Ranch Hot Springs surfaces at 135 degrees F deep in the Challis National Forest, requiring a nearly 4-mile hike from Sleeping Deer Road to reach this isolated thermal feature at 5,520 feet elevation in Custer County. The alkaline water carries a pH of 9.1 with 89 mg/L sodium, and the spring runs a full 94 degrees above ambient air temperature.
The hike follows a remote drainage through mixed conifer forest where lodgepole pine and Douglas fir dominate the slopes. The terrain is rugged and sparsely traveled, with the creek corridor providing the primary route. At the springs, hot water emerges from fractured rock and flows across mineral-stained ground before joining the cold creek. Steam is visible in cooler weather, rising through the canopy. The silence is striking, broken only by water, wind, and occasional bird calls. Snow piles over 9 feet deep here annually, burying the access route for much of the year.
This is a serious backcountry spring requiring a 4-mile hike each way with no maintained trail for portions of the route. Custer #1 Campground near the historic Custer townsite is 18 miles away with 6 free sites and a vault toilet. At 135 degrees F, the source water is dangerous and must be allowed to cool. The access season runs roughly mid-July through September. Carry bear spray, a map, and sufficient water. This area sees minimal other visitors.
Custer County contains one of the densest concentrations of hot springs in the continental United States, a product of the deeply fractured Idaho Batholith and the region's thin crust. Foster Ranch's chemistry mirrors other batholith springs: high sodium, elevated sulfate at 52 mg/L, minimal calcium, and strongly alkaline pH. The USGS recorded the source at 60 degrees C. The name references one of the ranching families that settled these remote valleys in the late 1800s, running cattle in the high meadows during summer months.
The water at Foster Ranch Hot Springs is alkaline (pH 9.1).
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This campground is along the Custer Motorway just above the townsite of Custer, a boom town during the late 1800's. Visit the museum, open 9 - 5 Memorial Day through Labor Day. Facilities include 6 sites with picnic tables and fire rings with cooking grills. Campground has a vault toilet and can accommodate trailers and motorhomes up to 32 feet long.
Foster Ranch Hot Springs sits at 5,520 feet above sea level, way above the mile-high mark. You may not feel the altitude strongly, but hot water still dehydrates you faster at elevation. UV is about 22% stronger than at sea level, so bring sunscreen and drink more water than you normally would.
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Foster Ranch Hot Springs is road-accessible, so you can bring more gear. The water runs hot, so extra hydration is key. Cold air temperatures make a changing robe or warm layers essential.
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