and very reliable, surging in 15 minute-long cycles of enormous discharge, then completely shutting down for a similarly long recharging period. This unusual and extremely rare hydrologic phenomenon is the outlet of a confined karst aquifer system hosted in the Madison Limestone. The intermittent spring lays in a deeply recessed canyon near Afton, Wyoming and requires a slow drive on a potholed gravel road and a short hike to reach.
Here is a well presented description of the spring, including an explanation of the mechanism at work: Geology of Wyoming - Periodic Spring.
Closer view of the modified spring orifice under flowing conditions. I recorded a time-lapse video of the spring over several cycles with a GoPro, tripod seen at left, and will post it here in a little bit.
UPDATED 19 September 2023: Here’s the time-lapse video of Periodic Spring through several cycles of discharge and recharge, each about 15 minutes long, shot over about 45 minutes. It’s magic!