Abstract

Post-eruption sedimentation rates from two basins draining Mount Pinatubo, Philippines, are compiled and extended through A.D. 2009 to compare short-term predictions with long-term yields. The Pasig-Potrero and Sacobia Rivers were covered by >1 km3 of thick pyroclastic flow deposits during the 1991 eruption of Mount Pinatubo, blanketing 30% of each basin with highly erodible sediments. Erosion of these deposits led to some of the highest sediment yields ever recorded. Yields peaked in 1991 and declined exponentially for the first decade following the eruption. Deposition on the aggrading alluvial fan shows that sedimentation rates from 2001 to 2009 were almost twice as high as an exponential decay model would predict, with rates that have leveled off and are no longer declining exponentially. Patterns of long-term sedimentation are similar to those of the North Fork Toutle River (Washington State, United States) at Mount St. Helens, with two phases of erosion and sediment export. In the first, erosion of hillslope tephra and carving of the initial valley network lead to high sediment yields that decline exponentially over 5–10 yr. In the second phase, continued valley widening and fluvial instability maintain high sediment yields for at least two decades. A comparison of post-eruption deposition with pre-eruption yields indicates that century- to millennial-scale sedimentation is dominated by sediment associated with discrete eruptive events.

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