Fluvial recovery from extreme sediment loading at Mount Pinatubo, Philippines
Professor Karen B. Gran, Dept. of Earth & Environmental Sciences, University of Minnesota Duluth
Seminar
Rue des Maraîchers 13, Room 001
One of the most devastating aspects of the 1991 Mount Pinatubo eruption was the hydrologic aftermath, with lahars occurring in major river valleys draining the flanks of the volcano for years after the eruption. On the east flank of Pinatubo, sediment yields reached > 3 million m3/km2/yr, with yields decaying exponentially for the first decade. This talk focuses on river evolution in the second decade following the Pinatubo eruption, at a time when most lahars had ceased and normal fluvial processes dominated sediment mobilization and transport. Field measurements of bed roughness, cross-sectional and long profile geometry, and bedload and suspended load transport were made on a series of rivers on the east flank of Pinatubo, with most effort on the Pasig-Potrero (P-P) River. Since the late 1990s, channels have coarsened as pumice and sand preferentially transport downstream. Coarsening occurred first in the upper basin during the dry season, extending downstream and lasting longer into the rainy season over time. By the end of the second decade, the upper basin of the P-P River contained gravel-bedded, single-thread channels even during the rainy season, reducing sediment fluxes downstream. On the P-P alluvial fan, channels remained sandy and braided during the rainy season for at least two decades, with armoring and incision occurring only as a seasonally-migrating downstream wave. Observed timing of the onset of vegetation recovery in the braidplain on the alluvial fan suggests that re-establishment of channel stability helps catalyze aquatic ecosystem recovery, with vegetation acting to restrict lateral mobility and encourage avulsive behavior. Comparisons of long-term fluvial recovery from Pinatubo and the North Fork Toutle River at Mount St. Helens show that after an initial exponential decay period, sediment loads tend to remain much higher than pre-eruption loads, driven by continued channel instability. The lesson learned at Pinatubo is that the hydrologic aftermath in the most impacted rivers may last much longer than previously anticipated, leading to on-going sedimentation problems lasting for decades. The amount of sediment moved out of the upper basin on the Pasig-Potrero River in the twenty years post-eruption is equivalent to over 750 years of sediment flux at pre-eruption rates, showing how rare events can come to dominate the sedimentary record.
