The heat flow into 7000-year-old Crater Lake caldera is dominated by hydrothermal processes. The average of the conducted heat flow value is 138¿121 mW/m2 (3.3¿2.9 HFU). The total heat flow is estimated to be between 669 and 1381 mW/m2 (16 and 33 HFU). Hydrothermal fluids penetrate into the lake floor to a depth of at least 1.5 to 2.0 km and possibly much deeper. Two thermal spring areas were discovered on the deep lake floor. These springs, and probably other as yet undiscovered ones, have a dramatic effect on the water temperature, circulation, and stratification of the lake. Dense thermal waters pond in the southwest basin. The deep waters of the east and northwest basin are warmed to the point that they convect, causing the lake to be vertically mixed. A hyperadiabatic temperature gradient results from this convection and possibly the stagnating effects of small quantities of suspended solids. Thermal fluids carrying dissolved solids cause anomalously high concentrations of chloride and sulfate in the relatively pure lake water.