Grand Prismatic Spring

Grand Prismatic Spring is Yellowstone’s largest hot spring, a pool hundreds of feet across in Midway Geyser Basin. The boardwalk brings you close to its blue water and colored edges; the separate overlook gives the broad view that explains the spring’s full ring. I’d do both if the uphill walk suits you, but the boardwalk alone is a worthwhile close-up.

The Grand Prismatic Overlook Trail starts at Fairy Falls Trailhead, about a mile south of Midway. It’s an easy 1.2-mile round trip with 105 feet of gradual climb. From above, you can see the spring’s full circular shape, which is hard to take in from the low boardwalk. The approach is sunny and exposed in summer. This is the view for a photo that shows the whole ring; use the boardwalk for steam and close detail.

Grand Prismatic is a hot spring, not a geyser. Its underground plumbing has no narrow constriction to trap pressure. Hot water rises, cools at the surface and sinks as hotter water replaces it, keeping the system circulating instead of building toward an eruption.

The deep water is near boiling and looks blue. Around the edges, microscopic heat-loving organisms live in bands of cooler water. Yellow pigments occur in hotter zones; orange, brown and green appear farther out as the water cools. Trillions of tiny organisms gather into the visible mats, so the color bands trace a temperature gradient around the pool. Steam can hide the colors; sunlight often makes them easier to see, but neither condition is guaranteed.

Ferdinand Hayden named it Prismatic after seeing its brilliant colors during the 1871 expedition. A trapper’s 1839 account by Osborne Russell may describe this same spring, but that identification is uncertain.

In the early 1990s, park geologist Roderick “Rick” Hutchinson and Livingston boatbuilder Mark Poppert made an 8-by-4-foot research boat called Little Dipper to sample Yellowstone’s hot springs. The stable little craft also helped researchers remove litter from the water.

The colored mats are only the most visible life in this extreme habitat. Mites, flies, spiders and plants also live among Yellowstone’s hot spring communities, in conditions that would be hostile to most life.