The 1993 rockslide at La Josefina, Ecuador, killed many people, displaced thousands, and dramatically altered a populated mountain landscape. The author's field research in this area, before and after the rockslide, yields new insights into the volume of sediment moving through tributary river systems in this Andean environment and into the ongoing interaction of human activity and local geomorphic processes. The rockslide, thought to have been triggered by human-caused as well as natural circumstances, moved 30 × 106 m3 of rock debris to the valley floor, blocking the flow of a major river and one of its tributaries. Behind the rockslide dam, a growing lake filled the valleys and trapped fluvial sediment. When a channel constructed in the debris released the impounded water 33 days after the rockslide, much of the sediment trapped upstream remained in place. The volume of sediment contributed by one upstream tributary river indicates that actual rates of sediment movement exceed those derived from studies and models of soil erosion in this area. High magnitude geomorphic events, rare in human experience, offer unusual opportunities to gain new perspectives on environmental processes. [Key words: sediment movement, landslide dam, catastrophic event, Paute River, Ecuador.]