Lake Todos los Santos is a lake located in the Los Lagos Region of southern Chile, 96 km northeast of the regional capital Puerto Montt and 76 km east of Puerto Varas, within the boundaries of the Vicente Pérez Rosales National Park. It has a surface area of 178.5 km² and a maximum depth of 337 m. The Lake's National Park status has ensured protection to its environment. The catchment is largely covered with old-growth Valdivian temperate rain forests. The present form of the lake is the result of glacial and volcanic processes.
The main tributary of the lake is the Río Peulla/Río Negro, next to the Peulla locality. Its outflow at the Petrohué locality gives rise to the Petrohué River, with an average outflow of 270 m³ per second. Even though the lake has a regulating effect, it is subject to water level variations that may exceed 3 m and reflect in the discharge at the outflow. At a short distance from the Petrohué locality, the river flows through the Petrohué Waterfalls.