How natural hot springs work

A hot spring is water that went into the ground cold and came back up warm. Everything on a spring page here — the reading, the band, the vintage — describes that water at the point it reaches the surface, which is not the same thing as the water in a pool.

Where the heat comes from

Two mechanisms account for nearly every spring in this directory, and they produce very different water.

Depth. Rock gets hotter the deeper you go. Rain and snowmelt work down through faults and fractures, pick up heat from the rock around them, and once the heated water is lighter than the cold water above it, it rises back along the same fractures and discharges at the surface. The USGS describes the round trip at Hot Springs National Park as roughly 4,000 years long, reaching 2,000 to 8,000 feet down, with water arriving at the surface at about 143°F on average. Most springs in the interior West work this way: no volcano involved, just depth and time.

Magma. Near young volcanic systems the heat source is much shallower and much hotter. The USGS Yellowstone Volcano Observatory puts the deep thermal fluids beneath the Yellowstone Caldera at about 350°C (662°F), boiling as they rise and the pressure drops. This is where the boiling-class entries and the steam vents come from, and it is also where the strictest rules live — thermal areas inside national parks are places to look at, not places to get into, and they ship here as look-only pages quoting the park's own rule.

Both mechanisms need the same plumbing: broken rock with a path to the surface. That is why the record is thickest in the faulted and volcanic West — the twelve states this site starts with, from Idaho and Colorado through Nevada and Oregon.

Why the source reading and the pool are two different numbers

The NCEI figure is taken at the orifice — the spot where water leaves the ground. Almost nothing about that spot resembles a pool. Between the two, water loses heat to open air and cold rock, spreads out and slows down, and usually mixes with whatever creek or river it lands next to.

The BLM says it plainly about Snively Hot Springs in Oregon: 190-degree emergent spring water, tempered by the cooling Owyhee River. Two facts, one place. Boiling Springs in the Boise National Forest reads 185°F at the source in the same list; Kirkham reads 149°F.

The gap between the two numbers is not fixed. It moves with air temperature, with how much water the spring is putting out that season, with the river's stage — spring runoff can swamp a pool that was pleasant in September — and with how the rocks are currently stacked, since the pools at primitive springs are piles of stone that people and floods rearrange every year. A source reading tells you about the spring's plumbing. It is not a forecast for the water you would be sitting in.

What the temperature classes on this site mean

Every spring here is placed in one of four bands. The band edges are this site's own; the readings inside them are quoted from the NCEI Thermal Springs List.

The edges are ordinary human reference points. Below 68°F the water is not meaningfully warmer than ordinary groundwater across much of the country, so it is not treated as thermal. 104°F is the ceiling most hot tubs are set to. Above 140°F the water does real damage on contact in seconds — see soaking safety and etiquette for what that means in practice.

A band is a description of water, not permission. Nothing on this site rates a spring as suitable to enter, and the band a spring falls in says nothing about whether anyone should be in it. The soak gauge on each spring page exists to print the record, not to hand out a verdict.

The B, H, and W codes

Of the 1,661 springs in the NCEI list, 1,480 carry a numeric temperature. 179 carry a letter code instead — 109 marked H for hot, 69 marked W for warm, one marked B for boiling — recorded by whoever visited without a thermometer reading to write down. Two entries carry neither.

The letters map onto the top three bands here: W to warm, H to hot, B to boiling-class. Where a record has only a letter, the gauge shows that band alone and prints no figure — inventing a number to fill the space would be inventing a fact.

What a 1981 reading does and does not tell you

The NCEI Thermal Springs List is a 1981 compilation of thermal springs across the United States, drawn from measurements made over the decades before it, and NOAA decommissioned the accession in 2025. It is the best public inventory of where America's springs are. It is also, in the strict sense, history.

What a reading tells you:

• A thermal spring existed at roughly that location when somebody visited.
• How hot the water was at the source on the day it was measured.
• Which band the spring belongs in — the coarse fact that survives the years best.

What it does not tell you:

• What the water reads today. Springs shift after earthquakes, drilling, drought, and nearby pumping; some cool, some heat up, some stop.
• What the pool reads, ever. See above — that is a different measurement in a different place.
• Whether the spring is still open ground. Since 1981 springs have been piped, capped, fenced, built over, and bought.
• Whether there is a legal way to reach it. That comes from the land manager, on each spring page, with the page linked and the date it was checked.
• Exactly where it is. Coordinate precision varies across the list; an entry recorded to two decimal places locates a spring within something like half a mile, which is a canyon, not a pool.

Water temperatures change with flow and season — test before entering, and never submerge your head in warm natural water. CDC guidance on Naegleria fowleri.

Common questions

Why does a spring read hotter on this site than the water felt in the pool?

The number on this site is the temperature recorded where the water leaves the ground. The pools people sit in are almost always downhill of that point, and the water loses heat to rock and air on the way and mixes with creek or river water when it arrives. How much cooler the pool runs changes with the season, the flow, the river stage, and how the rocks happen to be stacked that month.

What do the temperature classes on this site mean?

Four bands describe the recorded source water: cool below 68°F, warm from 68 to 104°F, hot from 104 to 140°F, and boiling-class above 140°F. The edges are this site’s, not the source list’s. They describe the water in the record and nothing else — a band is not a rating of a place, and no band means a spring can be entered.

What do the letters B, H, and W mean in a spring record?

They are the NCEI list’s own shorthand for entries recorded without a thermometer reading: W for warm, H for hot, B for boiling. Where a record carries only a letter, this site shows the matching band and prints no figure, because there is no figure in the source to print.

How old are the temperature readings on this site?

They come from the NCEI Thermal Springs List, a 1981 compilation of measurements collected over the decades before it, which NOAA decommissioned in 2025. A reading is one measurement taken at one spot on one day, printed here with its vintage attached. It is a fact about the record, not a report on the water today.

Next

• Springs near me

  The whole inventory, sorted by distance from where you are, with each spring's class on the row.

• Soaking safety & etiquette

  Scald reality above 140°F, the head-above-water rule and the agency behind it, and how not to be the reason a spring closes.

• Developed, primitive, or resort

  What the three kinds of hot spring are, which two this site covers, and how to tell them apart before you drive.

Or start with a state: Idaho, Colorado, or Oregon.