One of only three known cold-water intermittent springs in the world, the Periodic Spring is Star Valley's most genuinely unusual attraction. Tucked at the head of Swift Creek Canyon, the spring emerges from a limestone cliff face and gushes at full force for several minutes before stopping completely — a cycle that repeats roughly every 18 minutes during late summer and early autumn when water table levels are right. The mechanism is a natural siphon in the bedrock: an underground chamber fills until it reaches the lip of the siphon, then empties rapidly, only to fill again.
Getting there involves a pleasant 2 km walk up Swift Creek Canyon along a well-marked trail through tall cottonwoods and willows. The canyon itself is scenic enough to justify the trip even if the spring happens to be in its off-season (May through July, when snowmelt keeps the aquifer full and the siphon never fully empties, the spring runs continuously rather than intermittently). From Afton, take the road east up Swift Creek — the trailhead is clearly signed.