In hot water: Boiling River temps up; scientists have a few ideas why


If you think Yellowstone National Park’s Boiling River feels hotter this winter, you’re right. 

The temperature of the thermally heated water that flows into the Gardner River at the popular swimming hole has been increasing slightly since August of last year, park spokeswoman Amy Bartlett said Tuesday. 


Steam is pictured rising from the Boiling River where it intersects with the Gardner River in Yellowstone National Park, on Sunday.


In August, the temperature was approximately 134 degrees Fahrenheit. Over the past seven days, the temperature has ranged from about 136.5 to 139.5 degrees. 

These temperatures are too hot to swim in, explained park geologist Henry “Hank” Heasler Tuesday, but dilution with cold water occurs where the Boiling River meets the Gardner. He said 120 degrees is the temperature considered “scalding,” which can burn people, especially children. 

Park regulations prohibit swimming in hot springs and runoff that are of entirely thermal origin. The prohibition is two-pronged — one, thermal features are fragile and can be damaged by human activity; and two, hot springs are dangerous because water temperatures can spike suddenly. 

But park regulations allow swimming in many places where hot water meets a cold water stream — like where the Boiling River meets the Gardner. The swimming hole is located within the park between the North Entrance at Gardiner and Mammoth Hot Springs. 

Longtime locals swimming at Boiling River Sunday told Enterprise Managing Editor Justin Post the water seemed hotter than they’ve ever remembered it. A small cascade of hot water which in the past has been pleasant to sit under was unapproachably hot Sunday, Post said. 

And a loose rock berm, built up informally by swimmers over the years, used to be at the edge of a cold water “no man’s land” in the winter. On Sunday, swimmers were sitting on those rocks and even on the other side of them, Post said. 

Heasler, who has been with the park for about 12 years, said he and his staff aren’t absolutely certain what’s going on, but they are examining a couple of theories. 

One theory is cold water flowing into the Boiling River through underground sinkholes in the underlying travertine is being blocked or diverted somehow. 

A test of the hypothesis happened by accident last summer when water flowing into a sinkhole was temporarily diverted while park staff addressed a small area of invasive plants. 

While that water was diverted, the temperature of Boiling River increased, Heasler said. 

Heasler had another confirmation of this theory in January. On one of the coldest days of the winter, the Boiling River temperature climbed to 140 degrees. 

“I went and looked, and the sinkhole had frozen over,” Heasler said. 

Heasler compared the water flow to a kitchen faucet. Imagine turning on the hot water, he said. That water is pretty hot. Then turn on the cold water. The amount of water coming out of the faucet doesn’t change, but by turning on the cold water, the temperature drops. And conversely, by turning down the cold water, the temperature rises. 

Another explanation for the perceived increase in temperature could be that the Gardner River itself is at lower seasonal flow rates, which means less cold water to dilute the hot, Heasler said. Stream flows are naturally lower in winter when mountain water is locked up in frozen snowpack at higher elevations. Rivers run higher in the spring and summer when snow melts. 

And a graph showing Boiling River’s temperature from March of last year to the end of January of this year shows the stream’s lowest temperatures, between 105 and 110 degrees, occurring in late May-early June, the time of highest runoff. 

The water flow and temperature of Boiling River and the Gardner River are monitored at gauging stations placed by the United States Geological Survey. 

With continual, real-time data coming from the gauging stations, park safety staff have the information they need to “quantify” public safety decisions around Boiling River, Heasler said. Park staff use stream flow rates to determine when to close and open Boiling River each spring. 

When the Gardner reaches a high flow rate, rangers close the area due to high water. And when the flow rate drops, they re-open it. People can even watch online at home to assess when the swimming might be closed or about to open, Heasler said. 

Heasler said he could not address the question of whether park officials would ever consider closing Boiling River if the water temperatures remained high. 

“People who work with safety would have to decide that,” Heasler said, adding, “Yellowstone is a dynamic place and conditions do change.” 

The USGS provides its nationwide National Water Information System online for anyone to access, Heasler said. The web address is


Liz Kearney may be reached at