Box Canyon Springs

Box Canyon ends in a wall. No river comes in above it: the water that fills the canyon pours out of the rock at its head, about 180,000 gallons a minute, the eleventh-largest spring in the United States. It runs cold and so clear that the pool at the head shows blue, then drops over a twenty-foot fall and down a mile of canyon to the Snake.

Its source is far to the north and east, where the Big Lost and Little Lost rivers and a handful of creeks run out of the mountains and vanish into the lava fields around Craters of the Moon. The water moves southwest through fractured basalt in the Snake River Plain aquifer, which holds roughly as much as Lake Erie, and comes out along the north wall of the Snake's canyon here. The Nature Conservancy reckons the trip takes close to two centuries. How the canyon itself was cut is argued: a 2008 study concluded that a sudden flood carved it, not the slow undermining of the spring.

Cold, clean water in a desert does not go unclaimed. The valley's first commercial trout farm opened in 1909 outside Twin Falls, and the Magic Valley became the largest center of trout farming in the country. Box Canyon's owner held a right to divert much of the spring to a fish farm when The Nature Conservancy bought the land and water in 1999, with the state's help. It is now the Earl M. Hardy Box Canyon Springs Nature Preserve, a unit of Thousand Springs State Park, though some of the flow still goes to a hatchery. A trail runs along the rim and drops about 300 feet to the water.