The Río Lempa is the longest river in El Salvador, with a watershed covering 10,200 km2 (1,020,000 hectares), representing 48.5% of the country’s land area. The Río Lempa is one of the largest transboundary watersheds in the region, with 18,000 km2 distributed across El Salvador (56%), Honduras (30%), and Guatemala (14%). The river originates in Guatemala, in the Sierra Madre mountain range, then flows through southwestern Honduras, where it crosses El Salvador’s northwestern border, then flows southeast, where it flows into the Pacific Ocean.

The Río Lempa and its tributaries supply nearly 70% of El Salvador’s water resources, including the water supply for the San Salvador Metropolitan Area, the city of Santa Ana, as well as other secondary cities and rural communities.

The river is the main source of hydroelectric power generation, with four hydroelectric dams operated by the Executive Commission of the Lempa River (CEL). Four hydroelectric dams located in series along the river (including Guajoyo, Cerrón Grande, 5 de noviembre, and 15 de septiembre) have a combined generating capacity of 412 MW. Hydroelectric power accounts for one-third of El Salvador’s electricity generation (approximately 2.0 million megawatt-hours (MWh). 

The river is the source of some of the largest irrigation districts in the country, including Lempa Acahuapa, Atiocoyo Norte, Aticoyo Sur, and Zapotitán, with a combined coverage of up to 10,189 hectares. 

The watershed contains two areas protected by the Ramsar Convention on Wetlands, including Lake Güija and the Cerrón Grande Reservoir. These wetland ecosystems are important resources for migratory terrestrial and aquatic species, and both are threatened by forest cover loss, fires, invasive species, and severe domestic and industrial wastewater pollution . The basin also includes UNESCO Natural Heritage sites, such as the Apaneca -Ilamatepec and Trifinio-Fraternidad Biosphere Reserves .