The Yellowstone Volcano Observatory (YVO) was established as a collaboration between the U.S. Geological Survey and Yellowstone National Park "To strengthen the long-term monitoring of volcanic and earthquake unrest in the Yellowstone National Park region." Yellowstone National Park is underlain by a voluminous magmatic system overlain by the most active hydrothermal system on Earth. Tracking changes in water and gas chemistry is of great importance because anomalous fluxes might signal one of the earliest warnings of volcanic unrest. Because of the tremendous number, chemical diversity, and large aerial coverage of Yellowstone's thermal features, it remains daunting to monitor individual features that might serve as proxies for anomalous activity in the hydrothermal system. However, monitoring the chemistry of rivers provides advantages because their chemistry integrates chemical fluxes over a very large area and may reveal large-scale spatial patterns (Hurwitz and others, 2007, 2010). The ongoing monitoring and analysis of river solute flux is a key component in the current monitoring program. In addition, based on the application of the chloride-enthalpy method (Fournier, 1979), quantifying chloride flux in rivers provides an estimate of the total heat discharge from the Yellowstone volcanic system (Norton and Friedman 1985; Fournier, 1989; Friedman and Norton, 2007; Hurwitz and Lowenstern, 2014). Intermittent sampling of the large rivers draining Yellowstone National Park began in the 1960’s (Fournier and others, 1976), but more systematic sampling has been carried out since water year (1 October - 30 September) 1983 excluding water years 1995 and 1996 (Norton and Friedman, 1985, 1991; Friedman and Norton, 1990, 2000, 2007). Prior to 2001, only chloride concentrations and fluxes were determined. This report contains data collected between 2001 and 2014. In addition to chloride, the concentrations and fluxes of other anions of possible magmatic origin (fluoride, bromide, sulfate, and alkalinity) were determined, and several new sampling sites were established (Hurwitz and others, 2007). Beginning in 2007, the concentration of cations and trace metals were also determined. This Data Release supersedes USGS Data Series 278 v. 4.0 (Hurwitz and others, 2012). Following the protocols of Friedman and Norton (2007), water samples were collected from the major rivers once per month between November and February, every two weeks in March, April, August, September and October, and once a week between May and July, for a total of 28 samples per year for each river. As sampling resources decreased during the study period, however, the actual number of collected samples is less in many cases. River discharge at the time of sampling was obtained from the U.S. Geological Survey’s National Water Information System. Automated stream discharge measurements are made every 15 minutes, and the discharge at each of the rivers is measured manually several times each year to establish rating curves. At low discharges, differences between the manual and automatic measurements are typically less than 5%. At high flow rates, differences between the manual and automatic measurements can be higher than 5%. The list below includes all rivers, along with site abbreviations, that were sampled or have been sampled as part of this study (links are to the gaging stations): Boiling River near Mammoth (YBOI); Falls River near Squirrel, ID (YFAL); Firehole River near Madison Junction (YFIR); Firehole River near Old Faithful, WY (YFOF); Gardner River near Mammoth (YGAR); Gibbon River at Madison Junction (YGIB); Henrys Fork near Ashton, ID (YHEN); Madison River near West Yellowstone, MT (YMAD); Snake River above Jackson Lake at Flagg Ranch, WY (YSNA); Yellowstone River at Yellowstone Lake Outlet (YYFB); and Yellowstone River at Corwin Springs, MT (YYCR). Beginning in 2010, continuous specific conductance measurements at most river sites (except for the Boiling River and Yellowstone River at Fishing Bridge) have been used as a surrogate for chloride and other geothermal solute concentrations (McCleskey and others, 2012; McCleskey and others, 2016); thus substantially decreasing the number of water samples collected annually. References Fournier, R.O., 1979. Geochemical and hydrologic considerations and the use of enthalpy-chloride diagrams in the prediction of underground conditions in hot-spring systems: Journal of Volcanology and Geothermal Research, v. 5, p. 1-16. Fournier, R.O., 1989, Geochemistry and dynamics of the Yellowstone National Park hydrothermal system: Annual Reviews of Earth and Planetary Science, v. 17, p. 13-53. Fournier, R.O., White, D.E., and Truesdell, A.H., 1976, Convective heat flow in Yellowstone National Park: in: Proceedings of the 2nd U.N. Symposium on the Development and Use of Geothermal Resources, San Francisco, p. 731-739. Friedman, I. and Norton, D.R., 1990. Anomalous chloride flux discharges from Yellowstone National Park: Journal of Volcanology and Geothermal Research, v. 42, p. 225-234. Friedman, I. and Norton, D.R., 2000, Data used for calculating chloride flux out of Yellowstone National Park for the water years 1983-1999: U. S. Geological Survey Open-File Report: OF 00-0194, 48 pp. Friedman, I. and Norton, D.R., 2007. Is Yellowstone losing its steam? Chloride flux out of Yellowstone National Park, in Morgan, L.A., (Ed.), Integrated geoscience studies in the Greater Yellowstone Area: Volcanic, Hydrothermal and tectonic Processes in the Yellowstone Geoecosystem: U.S. Geological Survey Professional Paper 1717, p. 275-297. Hurwitz, S., Lowenstern, J.B. and Heasler, H., 2007, Spatial and Temporal Geochemical Trends in the Hydrothermal System of Yellowstone National Park: Inferences From River Solute Fluxes: Journal of Volcanology and Geothermal Research, v. p. 162, 149-171. Hurwitz, S., Eagan, S., Heasler, H., Mahony, D., Huebner, M.A., and Lowenstern, J.B., 2007, revised 2012, River chemistry and solute flux in Yellowstone National Park: U.S. Geological Survey Data Series 278, v. 4.0. Hurwitz, S., Evans, W.C., Lowenstern, J.B., 2010. River solute fluxes reflecting active hydrothermal chemical weathering of the Yellowstone Plateau Volcanic Field USA: Chemical Geology, v. 276, p. 331–343. Hurwitz, S. and Lowenstern, J.B., 2014. Dynamics of the Yellowstone Hydrothermal System: Reviews of Geophysics, v. 51. Ingebritsen, S.E., Galloway, D.L., Colvard, E.M., Sorey, M.L. and Mariner, R.H., 2001. Time-variation of hydrothermal discharge at selected sites in the western United States: implications for monitoring: Journal of Volcanology and Geothermal Research, v. 111, p. 1-23. McCleskey, R.B., Clor, L.E., Lowenstern, J.B., Evans, W.C., Nordstrom, D.K., Heasler, H.P., and Huebner, M.A., 2012, Solute and geothermal flux monitoring using electrical conductivity in the Madison, Firehole, and Gibbon Rivers, Yellowstone National Park: Applied Geochemistry, v. 27, p. 2370-2381. McCleskey, R.B., Lowenstern, J.B., Schaper, J., Nordstrom, D.K., Heasler, H.P., and Mahony, D., 2016, Geothermal solute flux monitoring and the source and fate of solutes in the Snake River, Yellowstone National Park, WY: Applied Geochemistry, v. 73, p. 142-156. Norton, D.R. and Friedman, I., 1985. Chloride flux out of Yellowstone National Park: Journal of Volcanology and Geothermal Research, v. 26, p. 231-250. Norton, D.R. and Friedman, I., 1991, Chloride flux and surface water discharge out of Yellowstone National Park, 1982-1989: U. S. Geological Survey Bulletin, B 1959, 42 pp.