Sunset over the Lempa river
|Countries||El Salvador, Honduras, Guatemala|
|- location||Olopa, Chiquimula, Guatemala|
|- elevation||1,200 m (3,937 ft)|
|- location||El Playón, Tecoluca, San Vicente, El Salvador|
|- elevation||0 m (0 ft)|
|Length||422 km (262 mi)|
|Basin||18,246 km2 (7,045 sq mi)|
|Discharge||for Cuscatlan bridge|
|- average||362 m3/s (12,784 cu ft/s)|
Railroad Bridge (FENADESAL) over the Lempa river, as seen from the Carretera del Litoral highway bridge (El Salvador)
Its sources are located in between the Sierra Madre and the Sierra del Merendón in southern Guatemala, near the town of Olopa. In Guatemala the river is called Río Olopa and flows southwards for 30.4 kilometres (18.9 mi) before entering Honduras and changing its name to Lempa river at . In Honduras it flows through Ocotepeque Department for 31.4 kilometres (19.5 mi), and crosses the border with El Salvador at the town of Citalá ( ) in the department of Chalatenango. The river continues its course for another 360 kilometres (220 mi) in El Salvador, flowing in a generally southwards direction until it reaches the Pacific Ocean in the department of San Vicente. The river forms a small part of the international boundary between El Salvador and Honduras.
The river's watershed covers 18,246 square kilometres (7,045 sq mi), of which 10,255 square kilometres (3,959 sq mi) (56 percent) is in El Salvador, 5,696 square kilometres (2,199 sq mi) in Honduras and 2,295 square kilometres (886 sq mi) in Guatemala. Forty-nine percent of El Salvador's territory is covered by the Lempa river basin, and 77.5 percent of the Salvadoran population lives in cities, towns, and villages that are in its basin, including the capital city of San Salvador.
There are several hydroelectric dams along the river. In El Salvador there is the Guayojo dam, the Cerrón Grande Hydroelectric Dam, the 5 de Noviembre dam, and the 15 de Septiembre dam which can be easily seen from the Pan-American highway.
- "Water Resources Assessment of El Salvador" (PDF). United States Army Corps of Engineers (USACE). 1998. Archived from the original (pdf) on 2009-01-09. Retrieved 2009-05-04.
- Hernández, Walter (2005). "Nacimiento y Desarrollo del río Lempa" (pdf). San Salvador: Servicio Nacional de Estudios Territoriales (SNET). Retrieved 2009-05-03.
- "Mapas de Recursos Hídricos". San Salvador: Servicio Nacional de Estudios Territoriales (SNET). Archived from the original on April 22, 2009. Retrieved 2009-05-03.