Water analyses are reported for 66 samples collected from numerous thermal and non-thermal (rivers and streams) features in the southwestern areas of Yellowstone National Park (YNP) during 2009, 2017, and 2018. Water samples were collected from sources near Boundary Creek, Bechler River, Falls River, Mountain Ash Creek, Upper Snake River, Spirea Creek, and Lewis Lake. These water samples were collected and analyzed as part of research investigations on the chemistry of Yellowstone's hydrothermal system and on the distribution of dissolved arsenic and mercury. Most samples were analyzed for major cations and anions, trace metals, redox species of arsenic, iron, nitrogen, and sulfur, and isotopes of hydrogen and oxygen. Radiogenic isotopes of strontium and tritium concentrations were also determined in selected samples. In addition, river and stream discharge data were obtained to determine the flux of chloride and other solutes from thermal areas in the southwest YNP.