Within hours of Mount Pinatubo's explosive June 15, 1991, eruption, heavy rains began to wash the ash and debris deposited by this explosion down into the surrounding lowlands in giant, fast-moving mudflows called lahars. By 1993, lahars had already caused more devastation in the lowlands than the eruption itself. Fortunately, a system to monitor and warn of lahars was rapidly established in the days following the eruption by the Philippine Institute of Volcanology and Seismology, U.S. Geological Survey, and other organizations. This system has saved hundreds of lives by enabling warnings to be sounded for most but not all major lahars at Pinatubo. U.S. bases have reverted to Philippine control since 1991. Subic Bay and Cubi Point are naval stations. 

1.  O'Donnell River
2.  Sacobia-Bamban River
3.  Pampanga River
4.  Pasig-Potrero River
5.  Marella River

The June 15, 1991, explosive eruption of Mount Pinatubo, Philippines, was the second largest volcanic eruption of this century and by far the largest eruption to affect a densely populated area. The eruption produced high-speed avalanches of hot ash and gas (pyroclastic flows), giant mudflows (lahars), and a cloud of volcanic ash hundreds of miles across. (U.S. bases have reverted to Philippine control since 1991.)