Measuring the thermal output of Yellowstone’s large magmatic system is not straightforward, as there are thousands of thermal features spread across 3470 square miles. One way to capture and integrate the contributions from this broad area is to monitor river chemistry, because thermal water discharge eventually enters a nearby river, which acts as a collection and delivery system. Nearly all the chloride in rivers that drain Yellowstone comes from emerging hot spring water heated underground by underlying magma. Monitoring river chemistry is therefore an important way to track the behavior and overall changes in Yellowstone’s hydrothermal system. By monitoring the chloride flux, the hydrothermal discharge and heat flux from Yellowstone can be estimated and variations (both short- and long-term) can be used to identify changes in the deep hydrothermal system, earthquake activity, geyser eruptions, and other natural events (like floods and the impacts of wildfire). The U.S. Geological Survey (USGS) and the National Park Service (NPS) have collaborated on Cl flux monitoring of the major rivers since the 1970s. In the past, researchers collected water samples from the major rivers in YNP, but funding restrictions, winter conditions, and the great distances between sites limited the number of samples collected annually. Beginning in 2010, specific conductance, which is relatively easy to measure and can be automated, has been used as a proxy for Cl. The use of specific conductance probes at the various monitoring sites enables a more consistent estimation of Cl flux. First posted - January 28, 2019 (available from author) Revised - May 6, 2020 (version 2.0; available from author) Revised - May 1, 2025 (version 3.0) NOTE: While previous versions are available from the author, all the records in previous versions can be found in version 3.0.