Spatial and temporal patterns in fluvial recovery following volcanic eruptions: Channel response to basin-wide sediment loading at Mount Pinatubo, Philippines

The June 1991 eruption of Mount Pinatubo, Philippines, was one of the largest volcanic eruptions of the twentieth century, emplacing 5-6 km ³ of pyroclastic-flow material on the flanks of the volcano. The combination of abundant, relatively fine-grained, easily erodible material and intense tropical rainfall led to numerous lahars immediately following the eruption. Even after major lahars ended, sediment yields in some basins remained orders of magnitude above pre-eruption levels. Using data collected from 1996 through 2003, we investigated five basins that experienced varying amounts of sediment loading in the 1991 eruption, from 1% to 33% of the basin area covered by valley-filling pyroclastic-flow deposits. From measurements of flow and bed characteristics made through time, we developed a general conceptual model for channel recovery following basin-wide sediment loading. Initially, finer-grained sediment and pumice are mobilized preferentially through selective transport. Once the bed is coarse enough for gravel-size clasts to interact with one another, clast structures develop, increasing form roughness and critical shear stress and inhibiting initial clast mobility. As sediment inputs continue to decline, the channel incises into valley bottom sediments, progressively armoring through winnowing. At Pinatubo, incision and armoring occur first as dry season phenomena due to reduced sediment inputs, eventually moving to year-round low-flow bed stability. Observed timing of the onset of in-channel biological recovery suggests that reestablishment of channel stability helps catalyze aquatic ecosystem recovery.