Dissolved noble gas data were collected using a portable mass spectrometer mounted on inflatable rafts on the Colorado River near Glenwood Springs, Colorado and the Virgin River near La Verkin, Utah. Data were collected approximately every 3 to 10 minutes for a suite of noble gases including helium (He), neon (Ne), argon (Ar), krypton (Kr), and xenon (Xe). Patterns in observed dissolved He concentrations were used to locate diffuse discharge of saline geothermal water, which commonly have elevated He concentrations. Diffuse groundwater discharge can be difficult to locate and quantify, and the dissolved He signals are a novel approach to understanding diffuse geothermal discharge to rivers. Dissolved He data were also used in a mass balance model to estimate the quantity of discharging groundwater.