There are over 10,000 hydrothermal features in Yellowstone National Park (YNP), where waters have pH values ranging from about 1 to 10 and surface temperatures up to 95 °C. Active hydrothermal areas in YNP provide insight into a variety of processes occurring at depth, such as water-rock and oxidation-reduction (redox) reactions, the formation of alteration minerals, and microbial (thermophile) metabolism in extreme environments, and possible indications of volcanic unrest. Investigations into the water chemistry of hydrothermal features, streams, and rivers in YNP have been conducted by the U.S. Geological Survey (USGS) and other earth-science organizations and academic institutions since 1883 (Gooch and Whitfield, 1888; Price and others, 2024). More recently, USGS researchers have sampled hydrothermal features in YNP at least annually since 1994 (McCleskey and others, 2014, and references within). In this Data Release, the chemical and isotopic analyses of 845 water samples collected beginning in 2009 are reported for numerous thermal and non-thermal features in YNP. This report combines water chemistry data presented in McCleskey and others (2014) with data collected after 2014. These water samples were collected and analyzed as part of research investigations in YNP on and as part of the Yellowstone Volcano Observatory monitoring plans (Yellowstone Volcano Observatory, 2006); arsenic, iron, nitrogen, and sulfur redox species in hot springs and overflow drainages; the occurrence and distribution of dissolved mercury and arsenic; and general hydrogeochemistry of hot springs throughout YNP. For most samples, data includes water temperature, pH, specific conductance, dissolved oxygen, and concentrations of major cations, anions, trace metals, alkalinity, sulfur redox species (hydrogen sulfide and thiosulfate), nutrients, silica, boron, arsenic and iron redox species, acidity, dissolved organic carbon, and hydrogen and oxygen isotope ratios. For select samples, tritium (3H), stable carbon isotopes of the dissolved inorganic carbon, and sulfur isotopes of sulfate are presented. In addition, chemical data for river, stream, and lake waters were obtained to determine input of different solutes from thermal areas throughout YNP. References Cited Gooch, F.A., and Whitfield, J.E., 1888, Analyses of waters of the Yellowstone National Park with an account of the methods of analysis employed: Bulletin 47, p. 84. McCleskey, R.B., Chiu, R.B., Nordstrom, D.K., Campbell, K.M., Roth, D.A., Ball, J.W., and Plowman, T.I., 2014, Water-Chemistry Data for Selected Springs, Geysers, and Streams in Yellowstone National Park, Wyoming, Beginning 2009: doi:10.5066/F7M043FS. Price, M.B., McCleskey, R.B., Oaks, A., Hurwitz, S., and Nordstrom, D.K., 2024, Historic Water Chemistry Data for Thermal Features, Streams, and Rivers in the Yellowstone National Park Area, 1883-2021: U.S. Geological Survey data release, https://doi.org/10.5066/P9KSEVI1. Yellowstone Volcano Observatory, 2006, Volcano and earthquake monitoring plan for the Yellowstone Volcano Observatory, 2006-2015: U.S. Geological Survey Scientific Investigations Report 2006-5276, http://pubs.usgs.gov/sir/2006/5276/. First posted - September 19, 2022 (available from author) Revised - March 4, 2025 (version 2.0) NOTE: While previous versions are available from the author, all the records in previous versions can be found in version 2.0.