R. Blaine McCleskey Maxwell L. Rudolph Sara Peek David A. Roth Melissa Schott-Atkins Michael Manga Kiernan F. Folz Donahue Mara H. Reed Jefferson D.G. Hungerford Shaul Hurwitz 2026 <p><span>The iconic Old Faithful Geyser in Yellowstone National Park, USA, has attracted a significant amount of research because of the relative regularity and impressive size of its eruptions. Numerous studies have included observations, measurements, and analyses that informed models of geyser eruptions. However, fundamental quantities, including the associated mass and heat discharged, remain poorly constrained. In April 2025 we measured the volume of water from 45 Old Faithful Geyser eruptions using a portable flume in an outflow channel and specific conductance measurements in the Firehole River. We used high-speed video to perform velocimetry, measured changes in water chemistry to calculate the volume of water evaporated along the outflow channels, and used thermodynamic calculations to estimate the volume of water erupted as steam and to quantify the geyser's heat output. The calculated average volume of water discharged by Old Faithful Geyser in each eruption is 27.9&nbsp;±&nbsp;9.4&nbsp;m</span>³<span>, with no relation between eruption volume and the length of the preceding eruption interval. Video analysis of the eruption's liquid-dominated phase yields similar volumes of 21–30&nbsp;m</span>³<span>. The calculated heat flow from the geyser is 2.2–2.4&nbsp;MW and the average annual discharge of chloride, fluoride, and arsenic are 63 tons, 3.9 tons, and 241&nbsp;kg, respectively. Average annual silica deposition rate on the geyser cone and along the outflow channels is 7 tons. This study provides a methodology for future studies at geysers worldwide and a baseline for monitoring future activity changes at Old Faithful.</span></p> application/pdf 10.1016/j.jvolgeores.2026.108624 en Elsevier Water volumes, heat flow, and solute discharge from Old Faithful Geyser eruptions, Yellowstone National Park, USA article