The Lerma River is Mexico's second longest river. It is a 750 km-long river in west-central Mexico that begins in Mexican Plateau at an altitude over 3,000 metres above sea level, and ends where it empties into Lake Chapala, Mexico's largest lake, near Guadalajara, Jalisco. Lake Chapala is also the starting point of Río Grande de Santiago, which some trea…The Lerma River is Mexico's second longest river. It is a 750 km-long river in west-central Mexico that begins in Mexican Plateau at an altitude over 3,000 metres above sea level, and ends where it empties into Lake Chapala, Mexico's largest lake, near Guadalajara, Jalisco. Lake Chapala is also the starting point of Río Grande de Santiago, which some treat as a continuation of the Lerma River. In combination, the two are often called the Lerma Santiago River.