Changes in the Position of the Zambales Shoreline Before and After the 1991 Mt. Pinatubo Eruption: Controls of Shoreline Change

Shoreline changes along the southern Zambales coast, both short-term - a few to several tens of years - and long-term - hundreds to a few thousands of years - have been determined from bathymetric and topographic maps, satellite images, space shuttle data, and aerial photographs. The dramatic increase of sediment input along the Zambales coast due to the 1991 Mt. Pinatubo eruption resulted in immediate, extensive, and rapid rates of coastal progradation at and adjacent to river mouths. The Bucao River mouth experienced the highest rates of progradation following the eruption, but rapid retreats also occurred. Furthermore, similar advances and retreats of this shoreline were also observed prior to the 1991 eruption; thus, the net change in shoreline position has been minimal. In contrast, progradation has been more pronounced along the discharge area of the Pamatawan and Sto. Tomas Rivers. This is surprising, given that their combined sediment yield is less than that of the Bucao River. Along the more southern segment of the coast, there has been greater progradation which may be attributed to the relatively gentler gradient of the adjacent shelf. Off the Bucao River, a submarine canyon taps the river mouth directly; thus, most of the sediments bypass the coast and shelf.The deltaic promontory that now characterizes the mouth of Sto. Tomas River was formed only after 1944. The delta formation cannot be due to the shifting of the river mouth because the Sto. Tomas River had been emptying at the same point even before the delta buildup. An increase in precipitation in the early 60's increased the river's discharge, which could have elevated the sediment yield leading to the delta buildup.Autocyclic changes in the distributary system of the Sto. Tomas alluvial fan redirected the flow of sediment to the Pamatawan River probably during the two episodes of eruption of Mt. Pinatubo prior to 1991. This resulted in the buildup of a delta much larger than the present day Pamatawan River delta.