Lake Chapala’s principal source is the Lerma River, which originates near Toluca in Mexico State and flows through the states of Michoacán and Guanajuato before entering Jalisco. The water entering Lake Chapala from the Lerma River is highly polluted with heavy metals and other toxic substances as a result of insufficient wastewater treatment by the many industries operating near the Lerma River. Additionally, many of the towns around the lake release their sewage and waste water into the lake without treatment. The mandated “Federal Zone” around the lake, where construction is prohibited, suffers increasing invasion by landowners.
Eleven major dams upstream of the Lerma River were constructed in order to divert water for industry, drinking water and irrigation. 25 % of the drinking water goes to Mexico City and 70 % to Guadalajara. In the 1970s Lake Chapala had a volume of 8.1 billion cubic metres [m³], 3.3 billion m³ is considered to be critical. The volume in summer 2003 was 1.2 billion m³ but due to heavy rainfall it increased to approx. 4 billion m³ at the end of the year 2003. In November 2009, the water level amounted to 5.5 billion m³.
So far the lake is internationally not well known. Even most Mexicans seem to have no specific knowledge about the importance and environmental situation of the lake. The wetlands of the River Lerma Delta are of great value and should be protected under national law. In February 2009, Lake Chapala was declared as RAMSAR site.