Periodic Spring
A rhythmic spring in Wyoming's Swift Creek Canyon near Afton, reported as the world's largest known intermittent spring, whose flow cycles on and off through a natural siphon in the Madison limestone; it has also served as Afton's municipal water supply since 1958.
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- Established
- 1894-08
- Location
- Swift Creek Canyon, near Afton, Wyoming
- Country
- United States
- Type
- Rhythmic (intermittent) cold-water spring
- Alternate name
- Intermittent Spring
- Ranking
- Reported as the world's largest known rhythmic spring
- Cycle (late August–November)
- Flows about 15 minutes, then stops for about 15 minutes
- Cycle (December–May)
- Flows about 4 minutes, then stops for about 26 minutes
- Geology
- Discharges from karst conduits in the Mississippian Madison Formation
- Water supply
- Afton's municipal supply since 1958
- Trail
- About 1.5 miles round trip, roughly 209 feet of elevation gain, on Bridger-Teton National Forest land
- Best viewing
- Late August through November, when water tables are lowest
Overview
Periodic Spring — also widely called the Intermittent Spring — is a rhythmic spring in Swift Creek Canyon, roughly four to five miles east of Afton, Wyoming, in Star Valley (Cowboy State Daily; Wyoming Water Research Center). It is reported to be the world’s largest known spring of its kind — a cold-water spring whose discharge switches on and off at fairly regular intervals rather than flowing steadily (Cowboy State Daily). It was discovered in August 1894 by Bruce Gardner, the great-great-grandfather of Olympic wrestler Rulon Gardner, while he was logging in the canyon (Cowboy State Daily). The spring sits on Forest Service land within the Bridger-Teton National Forest (Travel Wyoming), and it has doubled as Afton’s municipal water supply since 1958 (Wyoming Water Research Center).
The siphon mechanism
The leading explanation for the on-off cycling is a natural siphon. Geologist William W. Rubey proposed the model in 1972: a cave in the Madison Formation is connected to the spring orifice by a siphon-shaped passage, and during low flow the cave fills with groundwater until water spills over the siphon’s high point, driving out the air and starting a siphon that drains the chamber; when the water level falls below the siphon’s intake, air rushes in and breaks the siphon, and the remaining water drains away through cracks and talus until the cave refills and the cycle repeats (Geology of Wyoming). Shreve and Rubey later built a physical model of the system, and the treatment follows Peter Huntoon and James Coogan’s 1987 paper, “The Strange Hydrodynamics of Periodic Spring, Salt River Range, Wyoming” (Geology of Wyoming).
University of Utah hydrologist Kip Solomon has said of the siphon account, “We can’t think of another explanation at the moment,” and University of Utah studies in the mid-2000s found no gas content in the spring’s water — a result Solomon described as strong support for the siphon theory, since it shows the water was exposed to air underground rather than driven by dissolved gas (Cowboy State Daily). That finding distinguishes the spring from cold-water geysers, which are driven by gas; the spring is better understood as a non-geothermal intermittent spring (Geology of Wyoming). The Wyoming Water Research Center’s studies independently point the same way: water chemistry and temperature fluctuations indicate a karstic conduit system in the Madison limestone, with some spring snowmelt moving completely through the system in a matter of weeks — something the researchers say can only occur in a karstic system (Wyoming Water Research Center).
Geology and seasonal flow
The spring discharges from the upper part of the Mississippian Madison Formation, a regional aquifer of limestones and dolomites whose dissolution has produced caves and solution-widened fractures (Geology of Wyoming). Recharge comes from Madison outcrops along a fold called the Periodic Anticline near the spring and from larger outcrops about four miles east, with the Divide Anticline the probable major source because of its outcrop area, elevation head and snowpack (Geology of Wyoming). The research center’s sampling found the discharge swinging from a wintertime low of about 5 cubic feet per second to a summertime peak of up to 100 cubic feet per second, peaking between late June and mid-July, while releases from aquifer storage keep the spring flowing year round (Wyoming Water Research Center).
The intermittency visitors see is seasonal. From June through August the spring flows continuously at roughly 65 million gallons per day, as snowmelt overwhelms the siphon; from late August through November it runs about 15 minutes on and 15 minutes off, discharging about 13 million gallons per day; from December through May it flows about 4 minutes on and 26 minutes off at about 1.8 million gallons per day (Geology of Wyoming). The water is coldest — about 38°F — during the June–July peak, when dissolved solids are lowest at roughly 80 parts per million, and slightly warmer at about 43°F with roughly 120 parts per million in fall and spring (Geology of Wyoming).
A working water supply
The spring’s orifice is no longer natural. In the late 1950s the town of Afton tapped the spring for a water supply that needed no filtration or treatment; the small cave at the orifice was filled with blocks of talus and capped with a cement intake structure, and the water is piped to town (Geology of Wyoming). The University of Wyoming research brief confirms the spring has served as Afton’s water supply since 1958 and warns that contamination in the recharge area — uplands about six miles east of the spring — would reach the spring in a matter of weeks through the rapid conduit flow, while aquifer storage could keep the system from flushing clean for years (Wyoming Water Research Center). During the fall viewing season the city collects roughly 7 of the spring’s 13 million daily gallons, so a visitor watching the spring “come back to life” is seeing about half of its total volume run down the creek (Geology of Wyoming).
Visiting the spring
The trail is an easy draw: about 1.5 miles round trip with roughly 209 feet of elevation gain on Forest Service land, though the final section up to the spring’s mouth is steeper (Travel Wyoming; Geology of Wyoming). The canyon road’s first mile is paved and the next four are unpaved, and the road gate closes for winter on December 1 and reopens May 1 (Geology of Wyoming). Late August through November is the best window to see the on-off cycling, when regional water tables are at their lowest (Geology of Wyoming); Travel Wyoming and local media likewise point to late summer as the most noticeable season (My Country 95.5). The spring’s habit of stopping and restarting has earned it the nickname “the spring that breathes” (My Country 95.5; The Jerker). Atlas Obscura notes that the spring is said to flow for 18 minutes and stop for 18 minutes, and names the Gihon Spring in Jerusalem as another of the few comparable rhythmic springs reported worldwide (Atlas Obscura).
What this profile does not claim
No precise count of comparable rhythmic springs worldwide is stated here: Atlas Obscura and Wyoming local media say there are three, while Cowboy State Daily says “only a handful,” and no primary inventory was found to settle the figure. The “world’s largest known rhythmic spring” ranking is widely repeated in secondary reporting, but the specialist geology treatment hedges it as “purportedly” the largest, and no primary measurement basis for the ranking was found. Cycle times are seasonal rather than fixed — roughly 15 minutes on and off in the fall, 4 on and 26 off in winter, with general tourism coverage reporting 18–20 minute cycles — so no single interval is asserted. Discovery details beyond Cowboy State Daily’s 2024 account were not corroborated by a second source. A September 2022 Wyoming Business Report article on Afton’s water-supply concerns was found but sat behind a paywall and could not be read, so it was not used. The spring’s current flow regime beyond the May 2024 feature could not be confirmed from independently accessible sources.
Timeline
- 1894-08
- Discovered by Bruce Gardner — the great-great-grandfather of Olympic wrestler Rulon Gardner — while logging in the canyon.
- 1958
- The town of Afton taps the spring as its municipal water supply, a role it still holds.
- 1972
- Geologist William W. Rubey proposes the siphon model for the spring's on-off cycling, the explanation still in use.
- 1987
- Peter W. Huntoon and James C. Coogan publish "The Strange Hydrodynamics of Periodic Spring, Salt River Range, Wyoming" in the Wyoming Geological Association's 38th annual field conference guidebook.
- 1989–1991
- A University of Wyoming team samples the spring's discharge chemistry and temperature across three seasons and finds a karstic conduit system in the Madison limestone, with some snowmelt transiting the system in a matter of weeks.
- 1990
- Mark R. Blanchard completes a University of Wyoming master's thesis on the aquifer feeding the spring.
- 2000s (mid-decade)
- University of Utah studies find no gas content in the spring's water, supporting a siphon-based explanation over a cold-water-geyser theory.
- 2024-05
- Cowboy State Daily publishes a feature on the spring, the most recent independent feature found in this research.
What we could not verify
These claims came up in research and could not be confirmed. They are recorded here rather than published as fact or quietly dropped. If you hold a source that settles one, please tell us.
- Published sources disagree on how many comparable rhythmic springs exist worldwide: Atlas Obscura and Wyoming local media say three, Cowboy State Daily says "only a handful." No primary inventory was found to settle the count.
- The "world's largest known rhythmic spring" ranking is widely repeated but rests on secondary reporting; the specialist geology treatment hedges it as "purportedly" the largest, and no primary measurement basis was found.
- Reported cycle times differ by season and by source (roughly 15 minutes on/off in fall, 4 on/26 off in winter, 18–20 minute cycles in general tourism coverage). This profile presents the seasonal figures from the geology treatment rather than a single interval.
- Discovery details beyond Cowboy State Daily's 2024 account (August 1894, Bruce Gardner, logging in the canyon) were not independently corroborated by a second source.
- Whether the spring's flow regime has changed since the 1989–1991 and mid-2000s studies could not be confirmed from independently accessible sources; the most recent independent feature found dates to May 2024.
Sources
8 sources (3 tier 1, 3 tier 2, 2 tier 3), each opened and read against the claim it supports. Every source is independent of the subject; no first-party material is cited. See the source policy.
This profile rests on a University of Wyoming water-research brief (USGS-funded, tier1), Wyoming regional and local press (tier2), a specialist geology-of-Wyoming treatment that cites the primary Huntoon–Coogan and Blanchard literature (tier2), Wyoming's official state tourism office for visitor facts (tier1), and two curiosity/travel outlets used only for corroboration (tier3). The Wikipedia article on the spring was used only as a lead index and is not cited. Claims about cycle times are seasonal and vary across sources; they are presented as ranges rather than a single fixed interval. A September 2022 Wyoming Business Report piece on Afton's water supply was found but was paywalled and could not be read, so it was not used.
- Supports: The spring's location four miles east-northeast of Afton; Afton's water supply since 1958; the 1989 and 1991 sampling program and its findings (karstic conduits, snowmelt transit in weeks, year-round flow from aquifer storage); discharge of 5 cfs wintertime low to 100 cfs summertime peak; the recharge area six miles east and the contamination concern; the Blanchard (1990) and Huntoon–Coogan (1987) publications. Open original source
- Supports: The 1.5-mile trail with 209 feet of elevation gain; the spring's location within Bridger-Teton National Forest; the intermittency described as stopping for minutes at a time roughly every 20 minutes, most noticeable in late summer. Open original source
- Supports: The spring's setting in Bridger-Teton National Forest near Afton and the 1.5-mile trail, corroborating the visitor facts. Open original source
- Supports: The spring's location, its 1894 discovery by Bruce Gardner, its status as the world's largest known rhythmic spring, hydrologist Kip Solomon's explanation of the siphon mechanism, the mid-2000s University of Utah gas-content findings, and best-viewing guidance. Open original source
- Supports: The Rubey (1972) siphon model and the Shreve–Rubey physical model; the Huntoon–Coogan (1987) and Blanchard et al. (1990) literature; the Madison Formation setting, recharge areas and structural context; the late-1950s tapping of the spring and the cement intake; seasonal flow rates and cycle times; water temperature and dissolved-solids figures; trail access and the winter road closure. Open original source
- Supports: The "Spring That Breathes" nickname; the reported 10–18 minute flow periods; the late summer to early fall visiting window; the claim of three such features worldwide, flagged in this profile as varying across sources. Open original source
- Intermittent Spring in Afton Tier 3Supports: Corroboration of the 18-minute on/off cycle, the "one of three" claim, the Gihon Spring (Jerusalem) comparison, the siphon theory, and late summer as the most active season. Supporting use only. Open original source
- Supports: Corroboration of the "Spring that Breathes" nickname, the 12–18 minute cycle, the siphon explanation, snowmelt overriding the siphon effect, and the 1.5-mile round-trip trail. Supporting use only. Open original source
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