Lerma River

The Lerma River is a vital waterway located in west-central Mexico. It ranks as the seventh longest river in the nation. This river is part of a massive watershed that includes Lake Chapala. The federal government considers the Lerma-Lake Chapala basin the most important watershed in Mexico. When combined with the Río Grande de Santiago, it forms the largest river system in the country. The system begins on the Mexican Plateau and eventually reaches the Pacific Ocean through the Santiago river.

The river's journey begins at the Lerma lagoons near Almoloya del Río. These lagoons are located southeast of the city of Toluca. The water in these lagoons comes from natural springs. These springs rise from basaltic volcanics, which are volcanic rocks. These rocks flow down from the Monte de Las Cruces. This area sits between the Valley of Toluca and the Basin of Mexico. From these lagoons, the river flows a distance of about 700 kilometers. It winds through several Mexican states.

The river's path creates several borders between states. It forms a short border between Querétaro and Michoacán. Later, it flows west-northwest through the state of Guanajuato. The river then turns southward to separate Guanajuato from Michoacán. It also acts as a border between Michoacán and Jalisco. Finally, the river flows west-southwest of La Barca to empty into Lake Chapala. The water in Lake Chapala sits at 1,525 meters above sea level. Some people view the Río Grande de Santiago as a continuation of the Lerma River.

History and human settlement are deeply tied to this river. During the 17th and 18th centuries, large estates called haciendas were established along its banks. One notable example is the Atenco Ranch. This ranch was founded with bulls that belonged to Hernán Cortés. These bulls are considered some of the finest stock for bullfighting. Today, the river is surrounded by diverse locations. These include cities like Lerma and San Mateo Atenco. It also includes picturesque villages with cultural significance, such as Malinalco.

The river system is essential for agriculture and human life. While the river is not navigable by water craft, it is critical for irrigation. Currently, the river irrigates 820,000 hectares of land. An estimated three million hectares are used for agricultural production. Most of the people farming here are small farmers. Specifically, 52,125 out of 78,000 farmers in the watershed are classified as small. The river also provides municipal water for the Guadalajara and Toluca metropolitan areas. However, rural areas often face chronic problems accessing potable water from the river or its aquifers.

This basin supports a very rich ecology with many unique species. It is home to more than 100 native fish species and 19 introduced species. The most diverse native families are the splitfins and Neotropical silversides. Each of these families makes up about 10% of the native species. Other groups include poeciliids and cyprinids. Many species are endemic, meaning they are found nowhere else. Examples include various Chirostoma silversides and Algansea chubs. Sadly, some species like the Evarra chubs are already extinct. The area also hosts four endemic species of cambarid crayfish.

Pollution has been a major challenge for the Lerma River. Most pollution comes from discharging untreated or under-treated wastewater. Many industries operate in this area. These include meat, dairy, beverage, and chemical production plants. In the late 1980s, uncoordinated water policies made the situation worse. This failed to regulate water use among competing interests. In the 1990s, government programs helped improve water quality. By 1997, 45 treatment plants were operating with 70% efficiency. However, contamination levels became alarming again in the decade starting in 2000. In 2005, a sudden loss of oxygen caused thousands of fish to die in Pénjamo.