Lerma

Course

The Lerma River originates from the Lerma lagoons near Santa Cruz Atizapán, on a plateau more than 2,600 metres (8,500 ft) above sea level, and 24 kilometres (15 mi) southeast from the city of Toluca. The lagoons receive their water from springs rising from basaltic volcanics that flow down from Monte de Las Cruces. These are located between the Valley of Toluca and the Basin of Mexico.

The river forms the short border between the states of Querétaro and Michoacán then flows west-northwest through the state of Guanajuato. After turning southward, the river separates Guanajuato and Michoacán, and Michoacán and Jalisco before flowing, after a course of about 560 kilometres (350 mi), into Lake Chapala, 24 kilometres (15 mi) west-southwest of La Barca.

Ecology

The Lerma–Chapala–Grande de Santiago basin and associated systems are rich in freshwater fish, being the home of more than 100 native species and 19 introduced species. Among the natives, the two most diverse families are the splitfins and Neotropical silversides (each has about 1⁄4 of the native species in the basin), followed by the poeciliids and cyprinids (each has about 1⁄8 of the native species in the basin). Among these are many endemics, including several Chirostoma ("charales") silversides, several splitfins, a few Poeciliopsis livebearers, a few Algansea chubs, two Yuriria chubs, a few Notropis shiners, two Ictalurus catfish, two Tetrapleurodon lampreys, and more. Many of these are threatened, and a few Chirostoma silversides and all three Evarra chubs are already extinct.