The data
Every classification comes from real public data: the U.S. Geological Survey / NOAA Thermal Springs List for the United States, a national inventory of natural thermal springs with their locations and maximum surface temperatures. We use the machine-readable republication on PANGAEA (CC-BY-4.0). It is a historical reference inventory (1965–1980) — treat it as a starting point, not a live conditions report.
The water-temperature tiers
We classify each spring by its recorded maximum surface temperature. The thresholds are set for safety: U.S. health agencies (CDC/CPSC) cap safe hot-tub temperatures at 104°F / 40°C, so anything hotter is flagged as a hazard rather than a "soak."
| Tier | Temperature | What it means |
|---|---|---|
| Cool | below 86°F (30°C) | Tepid — more a cool dip than a soak |
| Warm | 86–99°F (30–37°C) | Pleasantly warm, below hot-tub heat |
| Ideal | 99–104°F (37–40°C) | Classic hot-tub range, up to the safe limit |
| Hot | 104–113°F (40–45°C) | Above the safe limit — too hot for many; burn risk |
| Scalding | above 113°F (45°C) | Dangerous — can burn in seconds; often a vent/geyser, do not enter |
Springs with no temperature on record are shown as "temperature not recorded" and are never assigned a tier — we don't guess.
Important limitations
- Surface, not pool, temperature. The figure is the maximum temperature at the source vent, which can be far hotter than any pool you could actually enter — the hottest records are geysers and steam vents.
- Historical & seasonal. The readings date to the 1960s–1980s. Real temperatures vary with season, rainfall, and exactly where you measure.
- No access or ownership data. The dataset says nothing about whether a spring is developed, public, on private or tribal land, or safe to reach. Many are undeveloped, closed, or dangerous.
- Not medical or safety advice. Verify legal access and current conditions with the land manager before visiting, test the water yourself, and follow Leave No Trace.
Data compiled 1965–1980; page generated 2026-07-13. Questions or a correction? Contact us.