Montgomery Since the 1991 eruption of Mount Pinatubo, the sediment yields from watersheds draining its slopes have been the highest ever recorded. In spite of this overwhelming sediment load, rivers inundated by pyroclastic flow deposits efficiently delivered almost half of the initial deposits to downslope alluvial/debris fans by 1996. Much of this transport occurs by lahars and hyperconcentrated flow, but at low to moderate flow the channels provide end-member examples of channel response to high sediment supply. A program of field measurements in 1997 and 1998 documented extremely high bedload transport rates even at low flow due to enhanced mobility of particles moving over a smooth, fine-grained bed. Measurement of flow velocities and depths, channel properties, and the size of both mobile and stable clasts in the Pasig-Potrero and Sacobia Rivers indicates grain-size-independent dimensionless critical shear stresses of 0.015 to 0.038, and Manning's n values of 0.017 to 0.024, well below values previously reported for steep mountain channels. The dramatic bed mobility and selective transport in these extremely sediment-rich channels indicate that changes in grain size, and bed roughness can increase transport capacity in response to high sediment supply. Dimensionless bedload transport rates in the Pasig-Potrero River are substantially greater than those previously reported for temperate rivers at the same dimensionless shear stress. Over the course of three field seasons, we propose to test a series of hypotheses for river response to changes in sediment supply using this ongoing natural experiment. Specifically, we will monitor channel response to the progressive decline in sediment supply to evaluate the hypotheses that: (i) bed coarsening and armoring is occurring in response to decreased sediment delivery from hillslopes and valley walls and will lead to a change in the character of bedload transport from selective mobility to threshold mobility; (ii) that recovery of an armored, threshold mobility bed in the Pasig-Potrero River will occur faster than in the Sacobia River due to differences in watershed recovery and relative transport capacity; (iii) that the magnitude and rates of changes in the bed surface texture in rivers draining the flanks of Mount Pinatubo track differences in initial sediment loading of their watersheds by pyroclastic flow deposits; and (iv) that pebble cluster formation is a mechanism of channel response to declining sediment load that, in turn, leads to changes in the style of bed mobility and bedload transport rates.
