Banff's well-known Upper Hot Springs pool has to be filled with tap water in the winter, and scientists don't know why the natural supply is drying up.

Parks Canada has been using municipal water to fill the pool during the winter at the 1930s era bathhouse for more than a decade.

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Every year tourists from around the world soak in the Upper Banff Hot Springs. (Carol Harrington/Canadian Press)

Geochemist Stephen Grasby said scientists are still trying to understand how the hot springs work.

"There are many parts of the system that we understand and make a lot of sense. And there are other parts that are completely baffling … so this is still a bit of a mystery to work on."

Canada's highest hot springs

Every year more than 300,000 people, including tourists from around the world, soak in the picturesque pool, with Mount Rundle as a backdrop.

The hot springs are the highest in Canada, at 1,585 metres above sea level.

Snow melt feeds the springs, trickling about 3½ kilometres in the earth before it is pushed back up to the surface through a large underground crack, deep in the earth's crust, called the Sulphur Mountain thrust fault. During the long journey, the water is heated, soaking up minerals.

Grasby said he doesn't think climate change is behind the low winter flows, but he needs more data.

"Students are regularly measuring the flows of all the different springs," he said. "The hope is once we get enough of that data we can start to generate some models that would help us better understand what is truly controlling and driving this system." 

The first problematic winter on record was in 1923, likely due to a lack of snow, but every winter since 1998 the hot springs have run dry. Some of those years experienced high precipitation and others low, Grasby said.

"It might just be taking a long time for the system to recover back to its normal self again."

Bathers not bothered

The water source is not kept secret from visitors in the winter, said spokeswoman Donna Cook.

"We have it in our brochure. We also have signs on site and it's explained on our website as well," she said.

Some bathers told CBC News this past weekend that they hadn't seen the signs and were unaware the pool was full of tap water.

"I'm surprised," said Mark Ross, a Winnipeg resident who has visited the hot springs dozens of times.

He said he would still recommend the springs to friends, even if it is not mineral water in the winter.

German Astrid Kalkbrenner didn't notice the signs either, but agreed a hot soak was still worthwhile.

"[In] the winter, it is just nice to go in a hot bath after an exhausting skiing day."

Parks Canada hasn't received many complaints, said Cook, who added that the real hot springs water should be flowing again by late May, in time for the height of the tourist season.

Parks Canada's other hot springs in the Rocky Mountain area are fed entirely by natural mineral water. Radium Hot Springs in Kootenay National Park is open daily, year-round, while Miette Hot Springs in Jasper National Park is open May to mid-October.