WATERSHED AND CHANNEL RECOVERY ON THE EAST FLANK OF MOUNT PINATUBO

Excess sedimentation from volcanic eruptions can drastically alter river systems. At Mount Pinatubo, Philippines, 5 to 6 km³ of pyroclastic material were erupted in the June 1991 eruption and emplaced over the nearby landscape. Immediately following the eruption, sediment yields reached record levels as massive quantities of sediment were mobilized and transported out of headwater basins, primarily through lahar flows. Our study examines the processes involved in watershed and river recovery from the volcanic disturbance from 1997 to the present, after the bulk of the lahar activity had ceased.

We are studying a series of five drainage basins that experienced different amounts of sediment loading, with pyroclastic flow material covering from <5% to >30% of each basin. Those basins with less sediment loading recovered much more rapidly. Because the devastation was not so widespread, river stabilization has progressed to the point where a functioning aquatic ecosystem has become reestablished. In basins with more widespread devastation, stabilization of the fluvial system is much slower, and the aquatic ecosystem has not yet recovered, with the channels devoid of algae, aquatic vegetation, insects, and fish. Our observations to date suggest that ecological recovery follows recovery of a stable channel bed.

To better understand the fluvial recovery process, we are focusing on the Pasig-Potrero River, which had more than 30% of the basin covered by pyroclastic flow material. We are monitoring adjustments to the channel both longitudinally and through time. Upstream depletion of fine-grained volcanic material is documented through a decrease in pumice content and increase in mean grain size on the bed. These changes occurred in upstream reaches first and have since progressed downstream through time. As the fine-grained material is evacuated and the bed coarsens, we have documented an increase in the critical shear stress required to mobilize sediment and a decrease in overall clast mobility as coarse grains begin to interact with one another and armor portions of the bed surface. Our on-going investigation seeks to link depletion of fine-grained sediment sources upstream to the observed changes in sediment transport mechanics.