Lake Chapala is the largest natural lake in Mexico, located 42 km south of the metropolitan area of Guadalajara. Hydrologically it belongs to the Rio Lerma Lago de Chapala Rio Santiago drainage system, one of the most important in Mexico. The main tributary R. Lerma supplies almost half of the water input, while the R. Santiago drains the lake water to the Pacific Ocean.

This region, the Mesa Central Region, is a highly unstable geological zone. The lake forms part of an east west oriented graben which is a Tertiary lake system where the majority of once existed lakes are now dry or almost dry. The geological history of the lake is poorly known. It is thought that the present lake basin and the R. Santiago outflow originated in the middle Pleistocene or the late Pliocene. The original drainage was probably from the west end of the lake directly to the Pacific Ocean, but uplifting blocked that flow establishing the present R. Santiago drainage. Terraces provide evidence of lake water level variation due to climatic changes during the Pleistocene.

The catchment area is large as compared with the lake area, amounting to 52,500 km². Sixteen percent of this area drains directly into the lake via small streams and runoff, while the remainder belongs to the watershed of R. Lerma. The climate is moderate, tropical and sub-humid with a single summer rainy season. Winter rains are less than 5% of the annual precipitation. Average annual evaporation (1,910 mm) greatly exceeds annual precipitation (781 mm in average).

Principal uses of the lake are irrigation, tourism, recreation and fisheries. In addition, it is the main water source for Guadalajara City with a population in excess of 4.5 million. The city used 2.108x1.0E+8 m³ of water in 1986, and water demands are increasing at a rate of 4% per year owing to urban, agricultural and industrial development. Principal industries in the lake's drainage basin are chemical, petrochemical and food processing. However, 93% of the water consumption in the basin are for agriculture. Predominantly untreated wastewaters are released to the R. Lerma. In the lower part of the river near the lake, much of the organic loading comes from pig farms (1).