Scientists of the Philippine Institute of Volcanology and Seismicity (PHIVOLCS) and the U.S. Geological Survey install an acoustic-flow monitor above a river valley filled with pyrolcastic-flow deposits from the 1991 eruption of Mount Pinatubo in the Philippines. When operational, the lahar-detection system provided scientists with a way to provide advance notification to the Philippine Office of Civil Defense and lowland-residents that a lahar was moving down a specific river valley.

Pyroclastic flows of the June 15, 1991, eruption filled canyons downstream from Mount Pinatubo to depths of up to 200 m (typically, 50-100 m) and stripped vegetation from valley walls and ridge tops over a broad area (note barren hill sides in background). Combined with intense typhoon and monsoonal rains common in the Philippines, increased runoff and erosion led to hundreds of lahars that swept from the pyroclastic flow deposits and buried many communities and extensive agricultural land (see case study). As shown by Mount Pinatubo and other recently active volcanoes, a lahar detection system is an important monitoring tool both during and after eruptions.