On a recent trip to Chico Hot Springs, my husband and I put on our bathing suits in the chilly concrete dressing rooms and walked barefoot and exposed across the icy walkways that many had walked before. The temperature was a frigid -5°. We gripped the ice-covered railings of the large pool and submerged ourselves in the warm, welcome embrace of the 110° mineral-rich water. The cold air over the hot pools was thick with fog. Before the warm water lulled us into a floating slumber, we jumped out into the ice-cold air, our bodies steaming, to freeze a little before returning to the comfort of the pool.
I had never put much thought into everything that had to happen for us and our fellow soakers to enjoy this unique activity. How is the water heated and where does it come from? Why do hot springs have such high mineral content? How long have people been soaking their bones in the healing waters? As I researched, I was surprised to learn that our state has 61 known hot springs—some private, some commercial, and some undeveloped—and they are mostly in Western Montana.
People (and animals!) have known about and taken advantage of the benefits of hot springs since the Paleolithic period. Indigenous peoples have used the hot springs in Montana for centuries to bathe and soak, cook, and do laundry. Some tribes considered the warm waters sacred. After the Indigenous peoples were pushed from their land, cowboys and settlers took over those spaces, and many of the hot springs sites were eventually converted into natatoriums and medical centers. Take a minute to look up the old Broadwater Natatorium in Helena. It was a spectacular building.
Hot springs are also known as geothermal springs and they are heated, well, geothermally. In Montana, this typically happens one of two ways. In Western Montana, water from the surface of the earth or from aquifers seeps down into geologic faults where it is heated in the depths of the crust. The less-dense heated water then rises back to the surface through another fault and forms the hot spring. Since hot water dissolves minerals in higher quantities than average or cold temperature water, hot springs tend to have high mineral content.
Hot springs are created a little bit differently in Eastern Montana, due to its unique geology. It is part of the Madison Group, a formation of carbonate rocks, mostly limestone, that extends across several states and at depths of up to a few thousand feet below the earth’s surface. Limestone is very porous, allowing rainwater to trickle down and reach the heated layers of the crust. Interesting side note: because the rock is soluble, it often turns into karst—a type of landscape where the water has eroded away the underlying rock and where caves often form. The Lewis and Clark Caverns are a well-known example of a cave formed in the Madison Group.
On our drive back from Chico, between Pray and Livingston, we noticed there seemed to be a lot of steam rising above the Yellowstone River. After a few days of hot springs soaking, naturally everything looked like a potential relaxing winter dip. A quick search of the United States Geological Survey website told us that the water temperature was about 32.5° that day, so jumping in would not have been wise. Turns out that what was happening was exactly what caused the fog over the pool at Chico: relatively warm water condensing in the colder air. I just didn’t realize that it could happen over water that was a frigid 32.5°! I think I'll stick to Montana's many hot springs for my soaking experiences.
Today’s Field Note was written in the Master Naturalist Course at the Montana Natural History Center. I’m Briana Wentworth for Field Notes, brought to you by the Montana Natural History Center, providing natural history education for schools and the public throughout Montana. For information on upcoming events and programs at the Center, call 406.327.0405 or visit our website at MontanaNaturalist.org.