High precipitation, steep slopes, small basin areas, and frequent flood events can induce high erosion rates on Oceania islands. These natural characteristics make watersheds much more vulnerable to anthropogenic perturbation. We analyzed long-term data sets (1950--1994) of suspended sediment loading from two gauges in Lanyang-Hsi watershed, a typical mountainous river in northeastern Taiwan. Prior to road construction that began in 1957, the annual sediment yield for the downstream gauge near the river mouth was in the range of 730--5400 t km-2 yr-1 with a mean sediment yield of 2800 t km-2 yr-1, which was 18 times higher than the global mean (150 t km-2 yr-1). However, following massive road construction projects, sediment exports increased more than tenfold for the downstream gauge and fortyfold for the upstream gauge at an altitude of 450 m, indicating an exacerbation of erosion induced by human activities. Such conditions of high export lasted for 2--4 years before the sediment yield returned to lower level. From 1957 to 1994 the mean sediment yield for entire watershed went up to 12,800 t km-2 yr-1; ~75% of that resulted from human perturbation. By comparing the sediment loads from the two gauges we concluded that the extra sediments mainly originated from the upper reach. A huge amount of the sediments apparently had resided in the middle reach; subsequent flood events with enough physical strength may resuspend and sweep the sediments to the sea gradually. Since the Taiwanese rivers represent the extreme conditions in correlation plots for the calculation of sediment export from mountainous rivers, one should exercise great care in distinguishing natural from perturbed conditions when using sediment yield data from Taiwan.