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Mammoth Hot Springs has one of the highest thermal water and gas discharges of the thermal areas outside the 0.6 Ma Yellowstone caldera. Thermal waters with surface temperatures of up to 73°C and calculated subsurface temperatures of about 100°C issue from nearly 100 hot springs scattered over a score of steplike travertine terraces that range in age from about 0.4 Ma to recent. Hydrologic and tracer tests conducted in 1989-1991 indicate that a very large flow, about 590 L/s, of thermal water from the Mammoth system enters the Gardner River. The heat flux from this system is comparable to that from the Norris Geyser Basin. The isotopic and chemical compositions of thermal waters and solutes from the Norris-Mammoth corridor can be interpreted to indicate a common magmatic source for heat and volatile solutes located near Norris for the entire corridor. However, the chemical and isotopic compositions of gases in general, and the distribution of ^3He/^4He ratios that could indicated proximity to a magmatic source in the corridor are obtained from Mammoth Hot Springs (R/Ra [less than or equal to] 8.4) and Norris Geyser Bason (R/Ra [less than or equal to] 9.0). A Norris to Mammoth flow path for the hydrothermal fluids at Mammoth, however, is likely precluded by the fact that fluids from all the six major thermal areas between them have R/Ra values that are appreciably lower than those from Mammoth Hot Springs.