The ongoing lahar crisis at Mount Pinatubo is related to the voluminous pyroclastic deposits emplaced on the volcano's upper and middle slopes during the June 15, 1991 eruption, and the limited sediment storage capacity of drainages and alluvial areas around the volcano. Radical alteration of the watershed hydrology by the eruption produced flows whose peak discharge was significantly greater than pre- 1991 eruption levels - despite normal rainfall intensity and duration. The combined sediment yield from eight major drainages by the end of 1995 amounted to more than 2 billion m³. This volume is several orders of magnitude higher compared to sediment yields measured from watersheds of comparable size elsewhere in the world. The large quantity of pyroclastic materials remaining on the slopes of the volcano indicate that excess sedimentation and flooding will be a long-term problem. As source materials are gradually depleted, however, the frequency and magnitude of lahars will gradually decline in the future but the potential occurrence of lethal lahars still remains, particularly in the event of extreme rainfall events.

Empirical data indicate that the projected decline in sediment yield in watersheds disturbed by volcanic eruptions is best approximated by an exponential decay model. Projected sediment yield from 1991 pyroclastic deposits over a period of 20 years for the Mount Pinatubo range from 2.8 to 3.6 billion m³. Additional sediment sources other than the June 15, 1991 pyroclastic flow deposits; watershed disturbances brought about by secondary explosions and pyroclastic flows; and an extreme rainfall event are likely to cause significant deviations from the projected sediment budget. Furthermore, the model does not take into account renewed eruption of the volcano which could provide new source materials, and reset the sediment - decay-curve. Despite these limitations the model provided useful insights, as well as the essential information required for formulating plans and strategies for lahar mitigation.

Lahar-mitigating strategies adopted for Mount Pinatubo include identification and qualification of lahar-prone areas; warning and evacuation; and construction of lahar containment structures (dikes). The first strategy is graphically represented by lahar hazard maps aimed at presenting severe, but reasonable predictions of areas vulnerable to lahars for planning rehabilitation and disaster responses. Actual distribution of lahar deposits attests to the accuracy of these maps in identifying areas immediately at risk. As the need arises, lahar hazard maps were revised to properly reflect actual field conditions. Lahar hazard scenarios in 1996 for the Pasig - Potrero and Marella-Santo Tomas rivers are briefly presented as case studies.

Combined with an effective warning system; properly designed and constructed lahar containment structures; and a disciplined and responsive local population, lahar hazard maps and scenarios could have been a effective tools in minimizing unnecessary loss of lives and properties. However, competing social, economic and political factors have hampered full adoption and utilization of available scientific information.