Cave Waters Tell a Story of Breathing Humans

A long-term study in a Utah cave shows that people’s exhalations have measurable impacts on cave waters and the growth of mineral formations.

By Kirsten Bahr, with Rebecca Weissinger, Chris Groves, Andy Armstrong, and Cami McKinney

A uniformed National Park Service employee with a helmet and light kneels on a cave floor overlooking a cave pool
A National Park Service physical science technician looks over Hansen Lake in Hansen Cave, Timpanogos Cave National Monument.

Image credit: NPS / Ryan Maurer

Visitors entering the caves at Timpanogos Cave National Monument are drawn into a world of white, mineral-crusted walls.

A multitude of fantastically shaped formations, called speleothems, emerge from every angle. Helictites cover the walls and ceilings like twisted crystalline snakes. Different metals combine with the minerals to create shades of lime green, lemon yellow, deep purple, and rusty orange. One of the most recognized features, a two-ton stalactite shaped like a heart, hangs from the ceiling. With so many wonders, it’s easy for visitors to overlook a small, crystal-clear pool named Hidden Lake, tucked just to the side of the tour route. But the pool doesn’t overlook them: long-term monitoring of its water chemistry shows humans can have a surprising effect on cave waters and their ability to precipitate cave formations.

The Heart of Timpanogos

How They Grow
Water absorbs carbon dioxide as it travels through soil. Cave formations grow when carbon dioxide dissolved in dripping water effervesces or outgasses into the cave atmosphere. This loss of carbon dioxide takes away the water’s ability to hold onto minerals like calcite, a calcium carbonate mineral. Calcite crystals precipitate and accumulate as a result, creating intricately shaped cave “decorations.” Image: the Heart of Timpanogos, a stalactite hanging from the ceiling of Timpanogos Cave. Image credit: NPS / Ryan Maurer.

Caves of “Unusual” Importance

Timpanogos Cave National Monument lies in rugged American Fork Canyon, 30 miles south of Salt Lake City, Utah. The Timpanogos Cave System is made up of three different caves: Hansen Cave, Middle Cave, and Timpanogos Cave. A committed group of local people, pioneered by the efforts of Martin Hansen in 1887, explored the caves then later took other people through them on tours. Early visitors climbed a rough trail that ascended 1,200 feet in one mile up the north-facing wall of American Fork Canyon. They explored the caves by candle and lantern light, finding Timpanogos Cave in 1913 and Middle Cave in 1921. On October 14, 1922, President Warren G. Harding signed Proclamation 1640, protecting the cave system as a national monument, citing its “unusual scientific interest and importance.”

In the 2009 season, there were 86,913 cave visitors over the course of six months. The caves’ popularity caused the visitor center parking lots to overflow onto the narrow canyon highway.

Since its discovery, the cave system has been wildly popular. In July 2009 alone, there were 25,435 visitors on 1,470 cave tours—an average of 820 people and 47 tours per day. In the 2009 season, there were 86,913 cave visitors over the course of six months. The caves’ popularity caused the visitor center parking lots to overflow onto the narrow canyon highway, and rangers couldn’t keep large tours from bumping into each other along the tour trail. The park instituted a cave management plan in 2013, intended to improve visitor experience and mitigate impacts of visitation on the cave environment. At the time, there were indications that the mere presence of humans affected cave features, but park scientists had no idea how much.

Heat, Light, and Crowds

Protected on a steep cliffside in the shadow of Mount Timpanogos, the cave system lies within a large natural area free of many external impacts. The most significant impact is the tour operation itself. The heat from visitors and cave lighting affects the caves’ microclimate. Sensors hidden along the tour route record the heat brought by every person who passes through the caves. At each tour stop, rangers turn on bright “feature lights” that stay on for four minutes before timing out. Before the park implemented the 2013 Cave Management Plan, tours were so frequent that one group often arrived at a stop just as another left. This required the feature lights to remain on for extended periods of time.

"It became apparent that the cave temperature was being artificially increased week by week to levels that would not return to normal before December."

Tours were limited to twenty people, but in practice, that number could be even higher. They began every 10 minutes, often creating a train of people stretching through the cave system. The caves were separated by tunnels, with doors designed to protect each cave’s microclimate. But the continuous line of visitors meant the doors remained open for a large part of each day, altering the air flow between the caves and drying them out.

When undisturbed, the Timpanogos cave system maintains an average annual temperature of 45 degrees Fahrenheit. But over time, the combined heat from tour lights, people, and people’s exhalations caused cave temperatures to increase. Temperatures often rose by half a degree during the day, then dropped back down at night. On busier weekends, temperatures would rise by one degree but only drop down a half a degree. This had the cumulative effect of increasing cave temperatures by a half degree each week, until the cave was six to eight degrees Fahrenheit warmer in summer than in winter. “It became apparent that the cave temperature was being artificially increased week by week to levels that would not return to normal before December, even when the cave tour season ended in September or October,” said Andy Armstrong, cave resource specialist for the park.

A graph of temperatures recorded from November 2018 to April 2022
A data logger in Chimes Chamber, located deep inside Timpanogos Cave, records temperature every 15 minutes. This graph shows temperatures recorded from November 2018 to April 2022. The temperatures spiked dramatically during the tour seasons, typically between May and October. During the 2020 COVID-19 shutdown, no one was allowed in the cave, not even park staff. The temperature in Chimes Chamber remained constant throughout 2020 and did not increase until tours began again in 2021.

Image credit: NPS / Kirsten Bahr

A Groundbreaking Management Decision

Under the 1988 Federal Cave Resource Protection Act, all caves on National Park Service lands must be managed by a cave management plan. In 2009, the Timpanogos Cave resource stewardship team proposed analyzing impacts from the cave tours for the park’s management plan. At that time, no other National Park Service cave management plan had directly addressed such impacts. And most other plans evaluated uses and impacts in undeveloped parts of caves, largely omitting management prescriptions for toured areas.

Regardless, the park’s resource managers felt they could not wisely manage and mitigate resource impacts without considering the tour operations. “With the tour trail visiting the majority of the Timpanogos Cave System, it seemed negligent not to address tours,” said Cami McKinney, program manager for resource stewardship and interpretation.

"With the tour trail visiting the majority of the Timpanogos Cave System, it seemed negligent not to address tours."

The planning team compiled data on parking, cave temperatures, visitor use patterns, tour timing, and tour frequencies. They also took into account emergency exit requirements and international occupancy standards (think fire code). From those considerations, the team prescribed a maximum of 16 people per tour, with a minimum of 15 minutes between tours. Jim Ireland, then superintendent, said, “We were making what we thought were the right management decisions based on the best information we had and our best professional judgment for the protection of the cave environment, but understanding that would be very hard to ever quantify in human time.”

A Hidden Record of Atmospheric Change

We live in a dynamic world, with changes that can be fast, messy, subtle, or slow. Long-term monitoring tries to find out what’s changing, how it’s changing, and how quickly. Every park in the National Park System with significant natural resources is part of an inventory and monitoring network that helps them answer questions like these. Timpanogos Cave belongs to the Northern Colorado Plateau Network, which also monitors 15 other national parks in four western states.

At Timpanogos Cave, long-term hydrologic monitoring began in 2007, when park staff began monthly cave visits to sample water chemistry and measure water quality in Hidden Lake and Hansen Lake. Hidden Lake is adjacent to the tour trail, only about eight feet away. Hansen Lake lies far from the tour trail, 300 feet from tours, where the only sound is dripping water and the darkness is total.

Instrument for measuring water quality surrounded by cave formations

Going Underground

It was May 2018 when I put on a caving helmet to enter the cold caves in Timpanogos for the first time as a physical science technician working for the National Park Service. I had collected and analyzed scientific data from caves before while working on my master’s thesis in geology, but nothing would quite prepare me for the experience to come.

At first, I was nervous that I would never learn to remember everything it took to collect and ensure the quality of all the data. Collecting data from the cave system is a complex operation. First, park Cave Resource Specialist Andy Armstrong and I calibrated our equipment, using standard solutions—ones with known concentrations—for conductivity and pH. We did this to make sure the sensors were reading and reporting correct values. In order to avoid contaminating slow-draining cave waters with our solutions, we did this in our lab the day before we headed to the cave.

After all the sensors passed our calibration tests, we got ready for the next day’s operation. We loaded the sensors, the paper field forms, and empty sample collection bottles into our backpacks. We also packed our packs with cave gear: a helmet, three sources of light, kneepads, a jacket for the cold temperatures, and caving gloves.

Early the next morning, we strapped on our backpacks and set off for the cave. We were facing a one-and-a-half mile, one-hour hike with a 1,092-foot elevation gain up the south wall of American Fork Canyon—not an easy task with our sampling equipment and gear. Once we reached the Hansen Cave entrance, we transferred all the equipment into a caving pack. This pack was designed with fewer external straps and buckles for easier movement through the cave. We put on our jackets and caving gear and headed in.

The cool air of the cave felt like the inside of a walk-in refrigerator. The light dimmed as if just before sundown. It’s called the “twilight zone,” a transition zone before the darkness. The light scattered and dissipated quickly outside the clear view of the entrance. Once we stepped out of the twilight zone, the light faded fast into total darkness. We could see nothing in front of us without artificial light.

We headed down the passage to reach Hansen Lake. Once we got there, we unloaded the pack, placed the sensors in the water, and let them equilibrate to temperature for about 10 to 15 minutes. When the instruments had stabilized, we recorded readings for pH, conductivity, dissolved oxygen, and temperature. Then we took a grab bottle, filled it up in the lake, then filled up the sample bottles from the grab bottle. These sample bottles would go to a lab later that day. We repeated this process at three lakes every month that the cave trail was free of snow and ice. Collecting samples from the three lakes took around two hours each time.

After we had collected all the samples, we headed out of the cave. We packed about five gallons—42 pounds—of water and the sensors in our packs for the trek back down the mountain. We returned to the park lab to filter a portion of the water and send it to a couple of labs in Utah. These labs would analyze the water samples for metals such as calcium and magnesium—both elements in limestone and dolomite, which the caves are made of. Several days later, we shared the analysis results with the National Park Service’s Northern Colorado Plateau Inventory and Monitoring Network. Image credit: NPS / Kirsten Bahr

Green cave formations against a background of white formations.
The mineral nickel creates a lime green color in the formations on the walls of Timpanogos Cave.

Image credit: NPS / Ryan Maurer

When we plotted 10 years of data from the cave lakes, several trends were clear. At Hidden Lake, pH was up, and alkalinity and calcium were down. Even nickel—the trace element responsible for the cave’s vibrant green formations—showed a pronounced decline. Hansen Lake also showed big trends. But they were different and sometimes opposite to the trends seen in Hidden Lake. Why were the cave pools changing so dramatically and so differently?

Led by network staff, we submitted our findings for publication, a process that would completely change our thinking. During peer review, scientists from the Kentucky Geological Survey and the National Park Service’s Gulf Coast Network saw that we had excellent data for studying the water’s carbonate chemistry. This would reveal the interactions between the cave’s water, its limestone formations, and carbon dioxide gas—the gas we exhale when we breathe.

"Fewer people breathing in the cave meant less carbon dioxide, not only in the cave air but in the waters of Hidden Lake!"

In trying to unravel the story held in these chemical data, the team pulled in a colleague from Western Kentucky University, Chris Groves. Groves related his reaction after he received an invitation from the team to study the data from the cave pools:

Analyzing and graphing up these data, overall things looked as one might expect, except for one thing: a clear and relatively sudden shift in the water chemistry, especially of Hidden Lake, had occurred in 2013, and whatever the cause might have been, continued to affect the water from that point on. This included the fact that the conditions in Hidden Lake were now conducive to the growth of mineral cave formations, where there were none before.

Deeply pondering for a few days but ultimately out of ideas about what might have caused this, I finally got on the phone with the crew, Rebecca, Andy, and Kirsten, to chat about this problem. The answer—immediately apparent—surprised and delighted us all! Unknown to me, the park had created a new cave management plan that year—fewer tours, and fewer people per tour. Fewer people breathing in the cave meant less carbon dioxide, not only in the cave air but in the waters of Hidden Lake!

A series of cascading chemical reactions were all consistent with this drop in carbon dioxide, including enhanced growth of cave formations in the lake. This subtle but significant change had been preserved in these data for all those years. And unknown over that same time, the 2013 cave management plan had had more impact that anyone even realized!

A plot showing the decrease in CO2 content following implementation of the 2013 cave management plan
Dissolved CO2 in Hidden Lake shows a sudden shift in 2013, when the cave management plan went into effect. The plan limited the size and frequency of cave tours. Fewer people breathing CO2 into the cave atmosphere meant less CO2 absorbed into the lake. This accelerated the growth of formations in Timpanogos Cave.

Image credit: From “Trends in Water Quality of Cave Pools at Timpanogos Cave National Monument, July 2008–September 2018,” National Park Service Natural Resource Report NPS/NCPN/NRR—2020/2181.

Lemonade from Lemons: The Pandemic Closure

In 2020, COVID-19 halted all tour operations in Timpanogos Cave National Monument. It was the first time in almost 100 years that the park’s tours ceased to operate during the summer months. The caves were also closed to staff. No people, no lights, no direct human influences. It was the first time in the history of the monument that the caves could be studied to find their natural summer levels of temperature, humidity, and carbon dioxide. Based on the long-term monitoring data, we immediately hypothesized that the closure would reduce carbon dioxide and enhance precipitating conditions in Hidden Lake.

As expected, carbon dioxide was markedly reduced. In the absence of visitation, cave formations grew before our eyes.

As expected, carbon dioxide was markedly reduced. In the absence of visitation, cave formations grew before our eyes. Our initial observations showed more calcite rafts (calcite precipitated out of the water) floating on the lake than in previous years, which got bigger and thicker throughout the year. In 2021, when the tours returned, those rafts began to shrink and break apart. We observed these changes through dedicated photo monitoring, but they were also readily visible to park staff. One ranger noted that at the beginning of the season, he would point out the rafts to visitors on his tour as they walked by. But further into the season, they got smaller, thinner, and harder to see.

The information we collected during the pandemic added to our long-term data indicating that the breathing of all those visitors had a striking impact on cave formations. Years of subtle water chemistry changes—bolstered by the pandemic study—showed the significance of unexpected impacts from the park’s 2013 cave management plan and its decision to limit cave tour sizes.

White cave formations
Helictites and stalactites in Timpanogos Cave.

NPS / Ryan Maurer

Separate Pieces Come Together

Many moving parts must work together to achieve a single scientific goal. In this case, the Northern Colorado Plateau Network and Timpanogos Cave National Monument put together a program to collect water from the cave lakes. The park supplied the materials and the personnel to collect the water. The network provided data management, statistical analysis, and reporting. And private laboratories analyzed the samples.

When the time came to interpret the results, university partners at Western Kentucky University lent a hand with carbonate chemistry calculations. The park had the visitation, planning, and facility management records to back up our findings. Without all these pieces coming together, it may have taken us many more years, if we could do it at all, to definitively see that the 2013 cave management plan had benefited the caves’ natural systems.

If we had only monitored the cave water for a few years, we wouldn’t have heard Hidden Lake’s own story of how humans influence the creation of cave formations.

All of this occurred over the course of 10 years, with many different people moving in and out of the same job positions. But because the sampling methods set up by the Northern Colorado Plateau Network were standardized, the results maintained their integrity no matter who was collecting the samples. And the program’s long-term nature was paramount. If we had only monitored the cave water for a few years, we wouldn’t have heard Hidden Lake’s own story of how humans influence the creation of cave formations.

Long-Term Monitoring Is Fundamental

Although fundamental for good management, long-term monitoring isn’t research. Research tries to answer specific questions, while long-term monitoring is more open ended. But long-term monitoring can still generate important questions for scientists to investigate, with valuable and insightful results. Take the case of Hansen Lake, which did not see the same tour-related changes that Hidden Lake did. Instead, Hansen Lake’s water chemistry trends over time mimicked those caused by outside air pollution. Why does a lake inside a cave feel the effects of air pollution? With further research, we could learn a lot about what Hansen Lake is trying to tell us.

“At [Timpanogos], a new cave management plan was designed to maintain more stable temperature and humidity within the cave,” said network ecologist Rebecca Weissinger. “Changes in water chemistry weren’t on anyone’s radar. But thanks to long-term monitoring data, we were able to notice that change. Now park managers can make more informed decisions about cave tour management in the future. They’ll be able to better advocate for maintaining cave conditions that promote cave decoration formation and an active, growing cave for future visitors to enjoy.”

Finding Balance

You might wonder why the park allows people inside the caves at all if they are affecting them. It’s true that when a human steps into an environment, they can change it. This is especially true in the austere cave environment. But there’s a difference between impacts and impairments. Monitoring programs help managers find that middle ground: how to preserve a resource while allowing visitors the opportunity to see what the natural world has to offer.

Show caves in national parks can inspire not only awe but also stewardship. They allow people to step into a world they may never otherwise get to see.

Show caves in national parks can inspire not only awe but also stewardship. They allow people to step into a world they may never otherwise get to see. Visitors can become advocates, and one tour may be the spark that a wild cave needs for its protection. If people come to a show cave and learn to appreciate it, they may be less likely to cause harm to caves elsewhere. They may even help educate others about the value of preserving them.

Sharing What We Learned

Scientific work at one national park can help other land managers understand their own resources a little bit better. But lessons learned from monitoring and research projects in one type of park, in our case a cave park, aren’t restricted to that park type. They can help us preserve a favorite tide pool in the ocean, a scenic lake overlook, or an ideal hiking trail. As Jim Ireland, now superintendent at Bryce Canyon National Park, said, “You don’t know what the data might eventually be used for, but you should be collecting baseline data as much as you possibly can.” You never know what amazing things you might discover.

Woman in black baseball cap, blue jacket, and white pants kneels next to a tan-colored dog with a red collar.

About the author

Physical Science Technician Kirsten Bahr works for the National Park Service at Timpanogos Cave National Monument. She is responsible for collecting, reporting, and analyzing data. She also operates and troubleshoots equipment for the park's cave program as well as for the meteorology and air quality programs. Kirsten enjoys exploring deep, dark, and sometimes (literally) ice cold caves. When she isn’t underground, she enjoys running agility courses with her dog, Zoe, or teaching her new, unique tricks. She also likes landscaping her yard in water-wise ways to make it more attractive to birds, pollinators, the local garter snakes, and small mammals like squirrels. Image courtesy of Kirsten Bahr.

Timpanogos Cave National Monument

Last updated: November 19, 2022