Aerial view of landslides and debris flows on the valley wall and terrace at Irlanda. For many parts of R?o Paez (channel in lower right), there were two sources of debris: (1) local landslides that slid in the river to form a brown-colored flow; and (2) the main lahar that arrived from upstream. This much larger black-colored flow progressively overran and incorporated the smaller brown flows to become what survivors called "the black wave of death."

The front of this main flow was about 10-m high and consisted of mud, boulders, and cascading tree trunks. It had a velocity as fast as 60-90 km/hour. The peak flow, following the front by several minutes, was as much as 40 m deep downstream of Irlanda.