BASIN-SCALE EROSION AT MOUNT PINATUBO, PHILIPPINES, AND IMPLICATIONS FOR LONG-TERM SEDIMENT YIELD FOLLOWING VOLCANIC ERUPTIONS

Basin-scale erosion patterns were compiled in the Pasig-Potrero/Sacobia River watershed of Mount Pinatubo, Philippines, for the decade following the 1991 eruption using repeat digital elevation models (DEMs), geomorphic terrain maps, and field observations. Net erosion measured with repeat DEMs declined from 5.1x10⁶ t/km²/yr in 1991 to 2.6x10⁶ t/km²/yr in 1993. Coupling repeat DEMs with geomorphic maps from Daag (2003) allowed us to subdivide net erosion into five categories based on erosional process: valley-bottom incision, valley-wall retreat, network extension, erosion from secondary explosions, and total erosion on pyroclastic-flow deposits from surface erosion and small-scale channelized flow (rilling and gullying). Sediment released from valley incision, widening, and extension dominated the sediment budget as early as the 1991 rainy season, with eroding valley walls continuing as an important sediment source through at least 2000. Secondary explosions were a significant mechanism for mobilizing sediment for 4-5 years after the eruption, further enhancing valley widening. Erosion on the pyroclastic-flow surface accounted for one-third of all erosion in 1991, declining in importance through time, particularly in the upper third of the watesrhed.

An exponential decay function effectively models sediment yield in the Pasig-Potrero/Sacobia basin throughout the entire first decade following the eruption of Mount Pinatubo. The decay constant is low (0.41), and sediment yields in 2001 were still twenty times higher than pre-eruption levels. A comparison with the Toutle River at Mount St. Helens shows a more complex pattern for long-term sediment yields, with yields decaying exponentially for 5-6 years, then leveling off 1-2 orders of magnitude higher than pre-eruption yields. If sediment yields on the Pasig-Potrero and Sacobia Rivers in the second decade after the eruption are maintained just one order of magnitude higher than pre-eruption levels, it will send an additional 14 million m3 of sediment downstream from 2001-2011. Field measurements of bed aggradation indicate that the Pasig-Potrero River has continued to maintain elevated sediment yields through at least 2005.