Particulate organic carbon export from a subtropical mountainous river (Lanyang Hsi) in Taiwan

Small mountainous rivers have received increasing attention in recent years because of their high yield of sediments and particulate organic matter. A one-year (1993-1994) study of the fluxes of the total suspended matter (TSM) and particulate organic carbon (POC) in different parts of Lanyang Hsi, a typical small mountainous river in subtropical northeastern Taiwan indicated that the mean TSM and POC yields of the whole drainage area were high (3,600 and 23 g m(-2) yr(-1), respectively) but that the yields of less disturbed (control) tributaries were only 1/30, of the mean yields of the whole area. Radiocarbon analysis by accelerator mass spectrometry gave very old apparent ages (>10,000 yr) to the POC in the main channel, indicating that >70% of the POC produced in the drainage basin was probably derived from kerogen in the bedrock. TSM yields for 1970-1991, calculated from historical records of runoffs and TSM concentrations, showed large fluctuations from year to year; however, the average yield after 1976 (8,335 g m(-2) yr(-1)) was higher than that before 1976 (3,127 g m(-2) yr(-1)) by a factor of 2.7, suggesting a significant increase in sediment production related to massive road construction in 1975-1980. The average POC yield for 1977-1994 was calculated to be 53 g m(-2) yr(-1). The very high yields of TSM and POC in recent years are attributable to human disturbances.