Rainbow Falls, Waterfall in Devils Postpile National Monument, California, US.

Rainbow Falls is the highest waterfall on the Middle Fork of the San Joaquin River, dropping 101 feet (31 m) inside Devils Postpile National Monument. The cliffs around it are platy rhyodacite, not granite: lava erupted from a vent east of the falls about 75,000 years ago in two pulses, cooling slowly and fracturing vertically. Undercutting has cut a small cavern at the base and pushed the falls roughly 500 feet upstream.

About 75,000 years ago, lava erupted from a vent just east of the present-day falls in two pulses. The first flow pooled in the Middle Fork of the San Joaquin drainage; the second insulated it so it cooled slowly and fractured vertically, while the layer above cracked horizontally. The falls now lie within Devils Postpile National Monument.

The forest along the trail shows signs of past wildfires, with charred trees standing beside fresh new growth. Visitors can observe how nature recovers after fires and gradually reclaims the landscape.

The trail leaves from stop 9 in Devils Postpile National Monument; the round trip is about 5 miles (8 km), not 3. The path back climbs steadily to the rim of the Middle Fork canyon, so carry water and wear shoes with good tread.

The return path from Rainbow Falls presents an unexpected challenge as hikers must climb back up the same elevation they descended. This demanding finish surprises many visitors with its difficulty.