Ezequiel Ramos Mexia Reservoir is a man-made lake in the Southern Andean Range, about 600 km away from Bahia Blanca. The lake was formed in 1972 by the damming of Limay River, which flows out from Lake Nahuel Huapi. The Limay Valley is formed by the 10-30 m thick layer of well-graded pebbles mixed with 30% of sand. The stratigraphic structure of the lake district consists of three kinds of sediments; a) Cretaceous sediments emerging continuously on both sides of the reservoir, b) Cenozoic sediments of conglomerates of variable sandy and clayey matrix, and c) recent sediments of fine to very fine sands forming dunes up to 7 m high on the northeastern side of the reservoir.

The lake water is soft, neutral to slightly alkaline, basically bicarbonate- calcic. The chlorophyll concentration and primary production are relatively low because of the low concentrations of nutrients (inorganic nitrogen and total phosphorus) and the short residence time. Therefore, the reservoir may be classified as an oligo-mesotrophic lake.