On 6 September workers began to partially drain Pinatubo's rapidly rising crater lake following the evacuation of ~40,000 NW-flank residents the previous day. Initially water flowed slowly at ~50 m3 per hour through the newly completed ~100-m-long spillway, but after workers increased the gradient at the canal's exit the flow rate increased to ~1,800 m3 per hour. According to news reports, over several days the draining aimed to release ~25% of the water in the crater towards the NW down the Bucao River to the South China Sea. Most residents were permitted to return to their homes on 7 September. According to reports on 10 September, the level of the crater lake continued to rise following the draining. Therefore, further preventative measures may be implemented.

Geological Summary. Prior to 1991 Pinatubo volcano was a relatively unknown, heavily forested lava dome complex located 100 km NW of Manila with no records of historical eruptions. The 1991 eruption, one of the world's largest of the 20th century, ejected massive amounts of tephra and produced voluminous pyroclastic flows, forming a small, 2.5-km-wide summit caldera whose floor is now covered by a lake. Caldera formation lowered the height of the summit by more than 300 m. Although the eruption caused hundreds of fatalities and major damage with severe social and economic impact, successful monitoring efforts greatly reduced the number of fatalities. Widespread lahars that redistributed products of the 1991 eruption have continued to cause severe disruption. Previous major eruptive periods, interrupted by lengthy quiescent periods, have produced pyroclastic flows and lahars that were even more extensive than in 1991.

Sources: Associated Press, CNN, Reuters, Reuters, Associated Press