A lahar travels down a river valley in Guatemala near the Santa Maria volcano, 1989. A lahar ( / ˈlɑːhɑːr /, from Javanese: ꦮ꧀ꦭꦲꦂ) is a violent type of mudflow or debris flow composed of a slurry of pyroclastic material, rocky debris and water. The material flows down from a volcano, typically along a river valley. [1]