Río Cuilco is a river in Guatemala. The river's sources are located in the Sierra Madre just north of Cajolá where it is called Río Blanco. The Río Blanco joins the Las Manzanas River which in turn joins the San Isidro River to form the Cuilco river. The Cuilco follows its largely northward course through San Marcos, and Huehuetenango, crossing the border with Mexico at 15.434156°N 92.106800°W (Río Cuilco), continuing its course northwards into the Presa de La Angostura, one of Mexico's largest artificial lakes.