In Yellowstone, Even Animals Sometimes Make Mistakes

Editor's note: Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Michael Poland, geophysicist with the U.S. Geological Survey and Scientist-in-Charge of the Yellowstone Volcano Observatory.

The recent death of a bison at Grand Prismatic Spring is an emphatic example of the danger posed by thermal areas in Yellowstone National Park. But there are some common misconceptions about what makes the hot springs so dangerous.

On the morning of Saturday, June 21, visitors at Grand Prismatic Spring in Yellowstone National Park observed a bison slip into a shallow part of the hot spring. Clearly alarmed by the hot water—up to about 89 °C (192 °F), which is just below boiling temperature at that elevation—the animal stumbled as it tried to get out and ended up stepping into an area of deeper water and quickly perished.

Yellowstone National Park officials decided not to remove the carcass. Such work would be dangerous and could cause extensive damage to the colorful bacterial mats that give Grand Prismatic Spring its name—that sort of damage can take more than a year to naturally repair itself. The bison’s body will break down quickly in the near-boiling water, and soon only the bones will be left.

Although rarely witnessed, it is not unprecedented that animals fall into hot springs. For example, an elk calf died while struggling to extract itself from mud pots in West Thumb Geyser Basin several years ago.

There are also animal skeletons visible in many hot springs, like Gentian Pool, Ojo Caliente, and the aptly named Skeleton Pool. Unexpectedly in 2022, University of California at Berkeley researchers found an elk skeleton in Doublet Pool, located on Geyser Hill near Old Faithful, when they put a camera into the spring as part of a study of hydrothermal activity. Similar incidents may have gone unwitnessed in the backcountry, or in winter or at night. Animals can obviously feel the heat of thermal areas through their feet or hooves, but that doesn’t necessarily mean they never make mistakes near hydrothermal features. Most of these wildlife fatalities probably take place when animals break through thin crusts on the edges of springs.

The unfortunate incident of the bison at Grand Prismatic Spring provides an opportunity to revisit a common misconception about hot springs in Yellowstone, and also emphasize how to safely enjoy viewing the thermal features in the park.

Many visitors to Yellowstone believe that all of the hot springs in the park are acidic (pH lower than 5), and that the water will cause an acid burn on contact and dissolve any animals or people that fall into the springs. But nearly all major hot springs and geysers in Yellowstone are actually neutral (pH of around 7) or alkaline (pH greater than 7), including almost all of the geysers and hot springs in the famous thermal basins along the Firehole River. Acidic features tend to be the fumaroles (gas vents), mud pots, and frying pans, which are all dominated by the release of acidic gases. One of the most acidic front-country features in the park is Sulphur Cauldron, in the Mud Volcano area. Fluids there have a pH less than 2, which is an acidity between that of stomach acid and lemon juice. Fortunately, acid concentrations are low, so mud and water from these features will not burn on contact, although it can cause skin irritation.

What kills most animals and occasionally people who fall into hot springs is not acidity, but temperature. Many hot springs are at or near boiling temperatures, and animal life will not survive for long when exposed, even if only briefly, to water that hot.

This is why staying on boardwalks in thermal areas of Yellowstone National Park is a requirement and is so important for visitor safety. Many thermal areas have thin crusts at the surface that overlie boiling waters or even hotter steam. Many injuries and fatalities to people have happened off boardwalk, when people have broken through these crusts, stepped on an unsupported sinter ledge, or stumbled into springs that were obscured or not obvious.

Yellowstone is a wild and dynamic place and can be hazardous. Fortunately, humans (and most animals) can recognize and avoid those hazards. So be sure to enjoy Yellowstone’s magnificent thermal areas from the safety of trails and boardwalks, and don’t forget to stay a safe distance from wildlife wherever you may encounter the park’s charismatic megafauna. For more tips on staying safe in Yellowstone National Park, visit https://www.nps.gov/yell/planyourvisit/safety.htm.