Lake Valencia is the largest natural freshwater lake of Venezuela. It is located in the central-north part of the country, which is the most densely populated. Lake Valencia lies on an east-west tectonic depression between two ranges of mountains: Cordillera de la Costa on the north and the Serrania del Interior in the south. The graben originated probably at the end of the Tertiary. As it developed, flowing waters of river Valencia were dammed and the lake was first established in the late Pliocene.

Peeters (1968) recognizes four different evolutionary periods for Lake Valencia. They are related to the alternation of humid and dry climate. Thirteen thousand years ago the lake basin was probably dry (Bradbury, et al., 1981). The lake drained into a tributary of Orinoco River until the beginning of the 18th century. However, due to a negative water balance the level of the lake declined very fast (7.5 m yr^-1, Bockh, 1956). The lake became endorheic about 250 years ago, when the discharge level (427 m) was exceeded due to desiccation. Intensive human intervention of the watershed and a reduction of groundwater flow have been mentioned for causing that rapid decline of the lake level. A minimum was achieved in 1976 (400.8 m above sea level). However, the lake level has recovered to 405 m with water provided from another watershed for urban consumption.

Incoming untreated wastewater from domestic, agricultural and industrial activities of about 2 million people contribute to eutrophication, contamination and salinization of the lake. The use of the lake as water source for domestic activities and for irrigation is restricted by the high salt content (electric conductivity ca. 2000 micro mhos cm^-1). Commercial fishing and recreation are also very limited by the precarious sanitary conditions of the water. Permanent algal blooms, high fish mortality, stench, etc. prevent the practice of aquatic sports and tourism (Q, 1, 2, 3).