Editor's note: The following is a column from scientists and collaborators of the Yellowstone Volcano Observatory. This contribution is from Blaine McCleskey, research chemist with the U.S. Geological Survey.

We often talk about monitoring Yellowstone. To most people, this implies tracking earthquake activity over time, or how the ground moves up and down, or how temperatures change due to thermal activity. But did you know that the river systems are also monitored? It turns out that tracking changes in river chemistry is a good way to identify overall changes in Yellowstone's hydrothermal system.

Chocolate Pots discharging into the Gibbon River/USGS

Yellowstone National Park was established due to its extraordinarily high concentration of hydrothermal features, including geysers, hot springs, mud pots, and steam vents. Groundwater interacts with the tremendous amount of heat from the magma body underlying Yellowstone to create more than 10,000 hydrothermal features. Monitoring these features is critical for managing the safety of visitors and for identifying changes in activity of the Yellowstone volcano, but tracking so many individual features distributed across 2.2 million acres is impossible.

Since the hot water from Yellowstone's thermal features ultimately ends up in Yellowstone's rivers, changes in river chemistry can be used to track overall hydrothermal activity. The most useful chemical indicator is in the chloride composition of the water, since hydrothermal waters have high concentrations of chloride. In fact, nearly all (95%) of the chloride in Yellowstone's rivers comes from thermal features. Thus, monitoring the chloride flux in Yellowstone's major rivers provides an overview of hydrothermal activity.

Chloride flux is the mass of dissolved chloride passing a point on the river during a set amount of time, and it is determined by multiplying the chloride concentration by the river discharge. Because all thermal features ultimately feed rivers, monitoring chloride flux provides an indication of hydrothermal activity not only in the major geyser basins, but also in remote areas and under lakes.

Sampling water from the Firehole River

The U.S. Geological Survey and National Park Service have collaborated on chloride flux monitoring since the 1970s. The chloride flux is monitored at several U.S. Geological Survey stream gage sites where stream discharge is continually measured. In the past, researchers collected water samples from these monitoring sites, but funding restrictions, winter conditions, and the great distances between sites limited the number of samples. Fortunately, however, the amount of chloride in river water correlates directly to the electrical conductivity of the water.

Beginning in 2010, specific conductance, which is relatively easy to measure and can be automated, has been used as an approximation for chloride. Continuous specific conductance measurements (every 15 minutes or 35,000 measurements per year) are made at monitoring sites along the Madison River, Firehole River, Gibbon River, Snake River, Gardner River, Falls River, Yellowstone River, and Tantalus Creek.

Thus far, no major changes in chloride flux have been recorded by the river monitoring system, suggesting that the overall hydrothermal activity at Yellowstone has been steady over the past few years. Any future changes, however, should be captured by the continuous monitoring now in place in Yellowstone's rivers. So the next time you hear about monitoring at Yellowstone, remember that it's not only about earthquakes and ground deformation, but also the waters that flow out of the park!

The use of specific conductance at the various monitoring sites enables a more consistent estimation of chloride flux and can be used to identify short-term changes in river chemistry because of geyser eruptions (like the recent eruptions of Steamboat geyser in the Norris Geyser Basin), rain events, or changes in thermal inputs because of earthquakes or other natural occurrences. In addition, the continuous specific conductance monitoring provides a cost- and labor-effective alternative to direct sampling, which required more people and generated less data.

Map of river chemistry monitoring sites in Yellowstone National Park.

The total chloride flux from Yellowstone can be estimated by monitoring just four rivers: the Madison River, Yellowstone River at Corwin Springs, Snake River, and Falls River. Of these four, the Madison River has the greatest flux—it accounts for about 46% of the total chloride flux out of the park. A quick look at a map of Yellowstone's river system will explain why. The Madison is fed by the Firehole and Gibbon rivers. The Firehole river contains about 75% of the chloride that ends up in the Madison because it drains the Upper, Midway, and Lower Geyser Basins. The remaining 25% in the Gibbon River comes largely from the Norris Geyser Basin.