About Periodic Spring
A few miles outside Afton, a mountain spring behaves like a faucet being turned on and off somewhere inside the rock. Periodic Spring surges from an opening near the base of limestone cliffs, flows strongly for several minutes, slows or stops, then begins again.
The U.S. Forest Service calls it one of only a few known cold-water periodic springs in the world. Unlike a geyser, no geothermal heating drives the cycle. The water remains cold.
The precise underground plumbing cannot be seen, but one leading explanation involves a natural siphon. Water may accumulate in a hidden reservoir until it reaches a level that starts siphoning through an underground passage. Flow continues until the reservoir falls below the siphon’s intake, then stops while the chamber refills.
Individual cycles vary, and the periodic behavior is affected by water conditions and season. The Forest Service notes flows ranging from roughly four to twenty-five minutes rather than promising a perfect schedule.
Visitors reach the spring by traveling from Afton into Swift Creek Canyon and hiking from the Periodic Spring Picnic Site. The trail is easy enough that the strangest part of the trip remains the destination: a natural spring that cannot seem to decide whether it is running.