Thermopolis Hot Springs State Park
♨️ The World’s Largest Mineral Hot Spring — Free Forever by Treaty with Chief Washakie, Rainbow Terraces, Wyoming’s Bison Herd, and 10,000 Years of Sacred History — Hot Springs State Park in Thermopolis, Wyoming with Big Spring (world’s largest mineral hot spring, ~2,200 GPM at 130°F), free public bathhouse (1896 Shoshone treaty mandate), Rainbow Terraces travertine formations, state bison herd (est. 1916, 640-acre pasture), swinging bridge over Big Horn River, Star Plunge, Hellie’s Teepee Pool, Legend Rock Petroglyph Site (900+ petroglyphs, 10,000 years), Wyoming Dinosaur Center, Wind River Canyon geology — Hot Springs County, WY
“Bah-gue-wana” — “Smoking Waters.” That’s what the Eastern Shoshone called this place for centuries before the U.S. government came looking. In 1896, Chief Washakie sold the hot springs — but with one condition: the water must remain free to the public forever.
That promise still holds. Walk into the state bathhouse today — no admission, no fee, no catch. Soak in 104°F mineral water that started as rain in the Owl Creek Mountains, traveled nearly 7,000 feet underground, and emerged here carrying the minerals of deep Wyoming geology.
Big Spring — The Numbers
| Measurement | Value |
|---|---|
| Flow Rate | ~2,200 gallons per minute — continuous, constant, year-round |
| Temperature | 128–130°F at the source (cooled to 104°F in bathhouse pools) |
| Depth of Source | ~7,000 feet underground — geothermally heated, not volcanic |
| Mineral Content | Sulfur, calcium carbonate, sodium, chloride, bicarbonate |
| Geological System | Thermopolis Anticline — water rises through fractures under artesian pressure |
| Classification | Non-volcanic hydrothermal system — distinct from Yellowstone’s volcanic springs |
The 1896 Treaty
The history of this park is inseparable from the Eastern Shoshone and Northern Arapaho. For centuries, the hot springs were part of the Wind River Reservation — a place of healing known as “Bah-gue-wana.”
| Year | Event |
|---|---|
| Pre-Contact | Eastern Shoshone use the springs for healing. Known as “Bah-gue-wana” — Smoking Waters |
| 1896 | Treaty negotiated. U.S. purchases 100 square miles including the springs for $60,000. Chief Washakie insists: the water must be free to the public forever |
| 1897 | Big Horn Hot Springs State Reserve established — Wyoming’s first state park |
| 1916 | State bison herd established in the park — 640-acre pasture |
| Today | Free bathhouse operates year-round. Washakie’s mandate honored for 129 years |
Rainbow Terraces
As the 130°F mineral water cools on its way to the Big Horn River, it precipitates dissolved calcium carbonate — building travertine formations layer by layer, century by century. The result is the Rainbow Terraces: cascading mineral shelves in vivid orange, white, and yellow.
The colors come from two sources: mineral chemistry (iron oxides, calcium carbonate) and thermophilic algae and plankton that thrive in the warm, mineral-rich water — the same organisms that create the colors at Yellowstone’s Grand Prismatic Spring.
| Feature | Details |
|---|---|
| Travertine Terraces | Actively growing. New mineral layers deposited daily as water cools |
| Color Source | Iron oxides (orange/red), calcium carbonate (white), thermophilic algae (green/yellow) |
| Boardwalk | Elevated walkway through the terraces — close-up views without damaging formations |
| Comparison | Similar formation process to Yellowstone’s Mammoth Hot Springs — but free to walk |
Where to Soak
| Facility | Cost | Features |
|---|---|---|
| State Bathhouse | FREE | Indoor and outdoor pools, 104°F. Year-round. No reservations. Chief Washakie’s mandate |
| Star Plunge | Paid | Indoor/outdoor pools, waterslides, vapor cave, kiddie area. Family-oriented |
| Hellie’s Teepee Pool | Paid | Indoor/outdoor soaking, waterslide. Quieter than Star Plunge |
Wyoming State Bison Herd
Since 1916, a resident bison herd has grazed a 640-acre pasture within the park. The herd is maintained at approximately 15 animals — calves are auctioned annually to manage population and genetics. The bison are often visible from the road and park trails, grazing on native grasses against a backdrop of hot spring steam.
Legend Rock Petroglyph Site
Thirty miles northwest of Thermopolis, a 1,600-yard cliff face tells a story that spans 10,000 years:
| Detail | Info |
|---|---|
| Petroglyphs | 900+ individual petroglyphs across 330+ panels |
| Age | 10,000+ years of continuous use — one of the oldest rock art sites in North America |
| Images | Anthropomorphic figures, elk, bighorn sheep, bison, abstract geometric forms |
| Cultural Connection | Dinwoody Tradition — ancestors of the Eastern Shoshone. Used for vision quests |
| Access | ~30 mi NW of Thermopolis. Self-guided or ranger-led tours (summer) |
Nearby Attractions
| Site | Distance | Highlights |
|---|---|---|
| Wyoming Dinosaur Center | In town | Full-size skeletons. “Dig for a Day” programs at active fossil sites |
| Wind River Canyon | ~15 mi S | Dramatic canyon drive through 2.5 billion years of exposed rock layers |
| Yellowstone NP | ~80 mi N | East entrance. Thermopolis is a perfect hot springs stop on a Yellowstone trip |
Best Time to Visit
| Season | Weather | Best For |
|---|---|---|
| Winter (Nov–Mar) | 15–35°F | ♨️ Hot springs in cold air — steam rising, snow on terraces. Magical contrast. Bison in winter coats |
| Fall (Sep–Oct) | 40–65°F | Cool air, warm water. Fewer crowds. Cottonwood color along Big Horn River |
| Summer (Jun–Aug) | 75–95°F | Full facilities, Legend Rock tours. Combine with Yellowstone |
| Spring (Apr–May) | 45–65°F | Terraces flowing strong, wildflowers. Bison calving season |
Frequently Asked Questions
Is the bathhouse really free?
Yes — forever. In 1896, Chief Washakie of the Eastern Shoshone sold the hot springs to the U.S. government for $60,000 with one condition: the water must remain free to the public. The State of Wyoming has honored this mandate for 129 years. Walk in, soak, leave. No fee.
Is this the world’s largest hot spring?
Big Spring flows at approximately 2,200 gallons per minute at 128–130°F — one of the largest mineral hot springs in the world. The water travels from ~7,000 feet underground through the Thermopolis Anticline.
Is this volcanic like Yellowstone?
No — this is a non-volcanic hydrothermal system. The water is heated by normal geothermal gradient at depth (~7,000 feet), not by volcanic magma. The geology is fundamentally different from Yellowstone, which sits on an active supervolcano.
What are the Rainbow Terraces?
Travertine formations built by mineral precipitation. As hot, calcium-carbonate-rich water cools, it deposits layers of travertine — the same process that creates Yellowstone’s Mammoth Hot Springs. The colors come from mineral chemistry and thermophilic algae.
Who was Chief Washakie?
Chief of the Eastern Shoshone for over 60 years (roughly 1840–1900). He was a skilled diplomat who negotiated the Wind River Reservation and ensured that the hot springs would remain free to the public in the 1896 treaty. He is one of few Native American leaders to have a U.S. military fort named in his honor (Fort Washakie).
Can I see the bison?
Yes — a resident herd has grazed the 640-acre pasture since 1916. About 15 animals, often visible from the road. Keep at least 75 feet distance.
What is Legend Rock?
900+ petroglyphs across 330+ panels on a 1,600-yard cliff face. Some are 10,000+ years old — one of the oldest rock art sites in North America. Associated with the Dinwoody Tradition and Eastern Shoshone vision quest practices. Located ~30 miles northwest.
How far is Yellowstone?
About 80 miles north to the east entrance. Thermopolis makes an excellent overnight stop — soak in the free hot springs after a day of driving through Yellowstone.
What is Wind River Canyon?
A dramatic canyon ~15 miles south of Thermopolis where the Big Horn River cuts through 2.5 billion years of exposed rock layers. Highway 20 runs through the canyon — geological time markers along the road explain each rock layer.
Is the water safe to soak in?
Yes — the bathhouse maintains water at 104°F. The mineral content (sulfur, calcium, sodium) is considered therapeutic. The water has been used for healing by Indigenous peoples for centuries and by the state bathhouse since 1897.
♨️ The World’s Largest Mineral Hot Spring — Free Forever
Chief Washakie’s 1896 mandate still holds: the water is free to the public, forever. 130°F mineral water from 7,000 feet deep, Rainbow Terraces rivaling Yellowstone, and a bison herd since 1916.
🪨 Legend Rock — 10,000 Years of Sacred Art
900+ petroglyphs on a 1,600-yard cliff. Vision quest site of the Dinwoody Tradition. One of the oldest and most significant rock art sites in North America, just 30 miles from the hot springs.
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