Actual measurements of advective heat flux from Yellowstone hot springs (Wyoming, USA) are seldom made, due to the difficulty of obtaining mass flow rates to support such measurements. Yet such measurements would provide important information that can be used to help evaluate the total thermal heat transport associated with the Yellowstone Caldera. Typically, discharge from thermal springs migrates through the shallow subsurface, making accurate measurement problematic. Here we present direct measurements of mass and thermal discharge from hot springs in the Lower Geyser Basin of Yellowstone National Park, USA. We added small amounts of nearly pure D2O to four springs in the Morning Mist Springs area that ranged in temperature from 74 to 95 °C and analyzed time-series δD samples to determine the volumes and discharge rates of the test springs. D2O was chosen to limit the ecological and/or visual impacts of other common tracers, such as NaCl or fluorescein dyes. We calculated spring volumes to range between 560 and 27,400 L and estimated mass and heat discharge as 0.08–1.25 L/s and 0.0189–0.312 MW, respectively. The volumes calculated by deuterium doping were larger in every case than those estimated by field inspection, suggesting that the volume participating in shallow fluid circulation is generally larger than is apparent from the surface. The heat flow data, when paired with conductive heat loss estimates in the vicinity of the springs, suggest that current estimates of thermal discharge at Yellowstone may underestimate heat loss from the caldera and offer insights on the rate of magma supplied by the mantle. Thermal flux estimates suggest that a minimum of 3.2–6.3 km3 × 10–2 of basalt magma enters the base of the crust annually.

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