Rhythmic springs are those springs that exhibit tidal characteristics. In other words, the water level of these springs rises and falls over a fairly regular time period. Sometimes the spring would stop flowing completely and start again after a couple of hours or minutes. The cause of this periodicity is not truly understood but there is a fairly sound theory.
It’s believed that upstream of a rhythmic spring there is a large underground cavern into which groundwater flows continuously. The outlet from the cavern is a narrow tube that first goes upwards from the near the base of the cavern, and then bends sharply down to the spring. As water fills the cavern, it also fills the tube. When the water level reaches the highest point in the tube, the bend, it starts to fall over the bend and towards the spring. This causes a siphon effect, sucking water out of the cavern until it empties. When the water level falls below the outlet, air rushes into the tube breaking the siphon and the spring stops. Water level in the cavern then starts to rise again and the cycle repeats.
Possible working principle of a rhythmic spring. Image credit: Bonacci & Bojanic
Rhythmic springs are a rare phenomenon. According to one study, there might be no more than a hundred rhythmic springs worldwide.
The largest rhythmic spring in the world is the Intermittent Spring, also called Periodic Spring, located in Swift Creek canyon near Afton, Wyoming, the United States. Another rhythmic spring, the Ebbing and Flowing Spring, is located in Hawkins County, Tennessee, near Rogersville. This spring has a period of 2 hours and 47 minutes. The spring's flow ranges from an indiscernible trickle to nearly 2,000 liters per minute.
Another well known rhythmic spring is the Gihon Spring Jerusalem. It was once the main source of water for the City of David in Biblical times. The spring was not only used for drinking water, but also initially for irrigation of gardens in the adjacent Kidron Valley which provided a food source for the ancient settlement. The spring flows from three to five times daily in winter, twice daily in summer, and only once daily in autumn. Because the spring was intermittent, a large reservoir called the Pool of Siloam, was constructed to store water for the town when the spring was not flowing.