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Lerma (lār´mä), river, c.350 mi (560 km) long, rising in Mexico state, central Mexico. It flows NW and W through Guanajuato state to Lake Chapala, crossing the part of the central plateau known as the Anáhuac. The river draining the lake and flowing NW through Jalisco state to the Pacific Ocean is generally called the Rio Grande de Santiago (c.200 mi/320 km long) but it is considered a continuation of the Lerma. The river system is extensively used for irrigation and hydroelectric power.