Rainbow Falls, on the Middle Fork of the San Joaquin River in California’s eastern Sierra Nevada within Devils Postpile National Monument, is the highest waterfall on that stretch of river, plunging 101 feet into a turbulent pool. Its frequent summer rainbows give it its name, but its deeper significance lies in the region’s volcanic history. About 75,000 years ago, two lava flows filled part of the river drainage and cooled in different ways, creating the fractured and platy rhyodacite visible in the surrounding cliffs. Those rock patterns still govern the falls’ erosion, causing undercutting and a gradual upstream retreat of about 500 feet.