Island Park Caldera
US-20
Island Park, ID
You can drive across the floor of an ancient supervolcano without realizing it, a broad forested basin ringed by low cliffs that are the rim of one of Earth's largest calderas.
Island Park is one of the strangest places you can drive through without noticing, because you are crossing the floor of an ancient supervolcano. The broad, forested basin is the Island Park Caldera, one of the largest and best-preserved volcanic calderas on Earth, and the low cliffs that edge the valley are the collapsed rim of eruptions almost beyond imagining.
The scale is hard to hold in your head. The caldera formed as the Yellowstone hotspot, the same deep heat source that now powers Yellowstone's geysers, erupted here long ago and collapsed the ground into an enormous bowl. Today that bowl is filled with lodgepole forest, spring creeks, and the headwaters of the Henry's Fork, a peaceful landscape sitting on top of a violent past.
What Guests Love
- Standing inside one of the largest volcanic calderas on the planet, hidden in plain sight
- The Henry's Fork and Harriman State Park, storied trout water and swan habitat within the basin
- Big waterfalls, springs, and lodgepole forest across a broad, gentle landscape
- A quiet, cabin-country gateway to the west side of Yellowstone
Worth Knowing
- The caldera is so vast and low-rimmed that it is hard to perceive from the ground; the geology is the wonder, not a dramatic crater
- This is high country with a short season; deep snow closes much of it in winter, when it turns into snowmobile terrain
- Summer brings mosquitoes to the wet meadows and springs; pack repellent
- Services are spread out along US-20, so plan fuel and supplies
Details
Island Park, ID
Learn about Island Park Caldera
A caldera on the hotspot track
The Yellowstone hotspot is a plume of heat rising from deep in the Earth. The North American plate drifts slowly southwest over it, so the hotspot has punched a chain of giant calderas across the Snake River Plain, roughly one every several hundred thousand years, like a blowtorch held under a moving sheet of metal.
Island Park is one of those calderas, formed by eruptions that ejected staggering volumes of ash and rock. Nested within the larger structure is the Henry's Fork Caldera, a younger collapse whose rim is easier to see. Yellowstone, just to the east, is simply the most recent stop on the same track, which means Island Park is a preview of a place millions of people visit without realizing they are neighbors on a geologic conveyor belt.
The Henry's Fork springs
The porous volcanic rock of the caldera acts like a giant sponge, storing snowmelt and releasing it through massive springs. The most famous is Big Springs, one of the largest spring complexes in the country, where crystal-clear water gushes from the ground fully formed to become the Henry's Fork of the Snake River.
Those steady, cold springs are why the Henry's Fork is such legendary trout water and why the meadows stay green and open even in winter. The river gathers itself here and runs south through Harriman State Park before dropping over Mesa Falls, all within the old caldera.
Wildlife of the basin
The caldera and the surrounding Greater Yellowstone Ecosystem are rich with wildlife. Moose browse the willows, elk range the forests and meadows, and trumpeter swans and sandhill cranes work the spring creeks, with a large share of the Lower 48's wintering swans centered on nearby Harriman.
Grizzly and black bears, wolves, and other Yellowstone wildlife move through this country too, part of one of the largest intact ecosystems in the temperate world. Birdlife is abundant along the rivers and lakes, and the whole basin feels like an antechamber to the park next door.
Seasons in the caldera
Summer and early fall are the seasons for fishing, paddling, and exploring the falls and springs, when the roads are open and the meadows green. Fall brings elk rut and crisp air, and the first swans down from the north.
Winter is long and deep here. Snow buries the basin and closes many roads to cars, and Island Park becomes a snowmobile and Nordic destination. If you are aiming for the springs, waterfalls, and wildlife, plan for the warm months.
Closest Boutels
The closest Boutel™ stays, so you can turn this stop into an overnight. Each is an independently owned, editorially selected place to stay.
Things to Do Near Island Park
Harriman State Park
Within the caldera
The old Railroad Ranch on the Henry's Fork, a wildlife refuge famous for trumpeter swans and technical fly fishing.
Mesa Falls
About 15 miles southeast
Two waterfalls on the Henry's Fork, the upper one a broad hundred-foot drop, on a historic scenic byway.
Big Springs
Near Macks Inn
One of the country's largest springs, where the Henry's Fork emerges fully formed and huge trout hover in the clear water.