Sediment supply and sediment storage in deltas of small mountainous rivers since 7 ka, west and southwest Taiwan

Abstract:

Taiwan's natural setting of high mountains, steep gradients, frequent earthquakes, erodible lithology, and heavy rainfall represents an ideal setting to determine the fate of sediment transported to the sea over various time scales. Sediment storage, remobilization, and cycles of erosion may be accelerated in this tectonically active region. Because small mountainous rivers draining west and southwest Taiwan (basin area about 7000 km², sediment transport 62.1–171.8 Mt/y) play a large role in coastal sediment supply, this study evaluated sediment dispersal and accumulation in the deltas of eight small mountain rivers since the last marine transgression (maximum flooding surface) about 7000 years ago (7 ka). The study area includes about 3700 km² of subaerial delta and 2400 km² of subaqueous delta built up by the Choushui, Peikang, Potzu, Pachang, Chishui, Tsengwen, Yenshui, and Erjen rivers. We analyzed digital elevation models derived from Shuttle Radar Topography Mission data and 85 radiocarbon dates of 65 borehole cores from these deltas, plus bathymetric and core data, to constrain estimates of deltaic sediment storage. Accumulation rates in the study area have been high, averaging about 0.58 cm/yr, since 7 ka, representing a total deltaic volume of about 200 km³ over that time span. The shoreline changes of the study area during the last century indicate that human effects (e.g., farming and urbanization) have increased sediment discharge over middle-late Holocene levels.