TWO DECADES OF CHANNEL EVOLUTION FOLLOWING THE 1991 ERUPTION OF MOUNT PINATUBO, PHILIPPINES

During the 1991 eruption of Mount Pinatubo, Philippines, pyroclastic flows deposited abundant sand-sized sediment in valleys, turning gravel-bedded single-thread streams into sandy braided rivers. This study focuses on channel evolution, bed armoring and organization, and feedbacks on sediment transport over two decades following the eruption on the Pasig-Potrero River. After lahars ceased in 1997, the bed has coarsened as sand and pumice preferentially transport downstream. Coarsening occurred first in the upper basin, during the dry season, extending downstream through time. The upper basin now contains gravel-bedded, single-thread channels even during the rainy season when sand inputs are high. Although the alluvial fan remains sandy and braided during the rainy season, dry season armoring and incision now extend >4 km downstream of the fan head. Incision occurs as a migrating wave, driven by differential transport rates in sand- vs. gravel-dominated reaches. Surface sand content varies from 3-7% in gravel-dominated reaches to >50% in sand-dominated reaches. In January 2010, this gravel-sand transition occurred in <200 meters.

Bedload and suspended load transport at the head of the alluvial fan (Delta 5 site) were compared during rainy season low-flow conditions in 1997, 2001, and 2009. This area remains a sand-rich depositional zone, maintaining high sediment transport rates in active braids. The locus of deposition has shifted downstream in the past decade. Aggradation rates remain high at Delta 5, at 0.9 m/year. The return of perennial vegetation to the braidplain between 2007 and 2009 has focused active braiding (and thus aggradation) into a smaller valley width, limiting depositional space. This imbalance has created a 1% lateral grade across the braidplain and should force the main flow back into vegetated areas, leading to complex interactions between dynamic braiding and vegetation.