The lake known as O'Higgins in Chile and San Martín in Argentina. It has a surface area of 1,058 km², an elevation of 250 metres above mean sea level, and a shoreline length of 525 km. Viewed from above, the lake consists of a series of finger-shaped flooded valleys, of which 554 km² are in Chile and 459 km² in Argentina, although sources differ on the precise split, presumably reflecting water level variability.
The lake is the deepest in the Americas with a maximum depth of 836 metres near O'Higgins Glacier, and its characteristic milky light-blue color comes from rock flour suspended in its waters. It is mainly fed by the Mayer River and other streams, and its outlet, the Pascua River, discharges water from the lake towards the Pacific Ocean at a rate of 510 m³/s. The O'Higgins Glacier flows eastwards towards the lake, as does the Chico Glacier. Both of these glaciers are part of the Southern Patagonian Ice Field which extends for approximately 350 kilometres in a north-south direction to the west of Lake O'Higgins.