¡@¡@Sediment loads of Taiwan rivers have increased in recent years because of frequently recurring typhoons. In addition to the dry and wet seasons causing sever erosion and deposition of the riverbed, the long-term records from 1967 to 2009 of suspended sediment concentration and daily discharge observed by the Water Resource Agency in Taiwan Rivers were used in this study. ¡@¡@To understand the sediment deposition or erosion in the rivers, we need to find how the discharge and sediment load interact with each other under the natural and human influences. The hydrological data we used in this study were from Water Resources Agency¡¦s annual reports, including the Danshui River, the Lanyang River, the Zhuoshui River, the Beinan River, and the Gaoping River. ¡@¡@High turbidity happen when sediment concentration exceeds 40 g/L. Judging by this threshold, taking upstream stations of the Zhuoshui River for example, the Nei-Mao-Pu Station (1973-2009) reached this threshold 6 times, Bao-Shih Bridge Station (2004-2009) 19 times, and Yun-Feng Bridge Station (1995-2009) 19 times. But Yen-Ping Bridge Station (2004-2009), which is at the middle part of Zhuoshui River, has never reached the threshold, became the erosion rate and sedimentation rate are balanced here. Chi-Chou Bridge Station (2000-2009) at the downstream reached the threshold for 11 times too. ¡@¡@According to the river graded profile, in the Danshui River, obvious erosion occurred at the Liu-Kwei Bridge Station in 2000, the Po Bridge Station in 2001, and San-Hsia (2) Station in 2004 and 2007. In the Zhuoshui River, significant deposition took place at the Yun-Feng Bridge Station in 2006 and 2010. In the Gaoping River, deposition occurred at the A-Chyi-Ba Bridge Station in 2000, while erosion occurred at Liu-Kwei Station in 2001 and at the A-Chyi-Ba Bridge Station in 2007 and 2008. ¡@¡@Above results show that the major sediments are deposited upstream, forming a temporary sediment storage source region, such as at Li-Lin Bridge Station, Liu-Kwei Station, and Yun-Feng Bridge Station at the Gaoping River, all of which have reached the threshold when a typhoon comes. Investigating daily rainfall data from Taichung Weather Station in 2005, we find that besides typhoon influences, when the rainfall is exceeding 200 mm High turbidity events would occur in the Jhoushuei River. ¡@¡@According to our study, we find that most sediment deposits at the upper reaches, forming temporary sediment storages, so when a typhoon comes, we can see the sediment downstream is often mixed with the sediment coming from upstream storages. That is because the slope in the upper reaches is steeper, and heavy rain makes the temporary deposit easy to erode. On the contrary, the slope in the lower reaches is more gentle so it is easier for sediment to deposit, which is the reason why the stations at downstream often reach the threshold. When the extreme weather occurs, the temporary sediment storages along the entire river course become the source regions to release the sediment out to the sea.

The purpose of this research is to understand the sinking particle dynamics in the Gaoping Submarine Canyon (GPSC), the change of their geochemical character, and their causal relationship with dynamic parameters. Also this research inquires into the significance of sedimentary environment, transport process, and the influence of non-tidal actions (turbidity current) in the sedimentary environment. The field experiments including LADCP moorings, T6KP(1/10/-3/20), and T7KP (7/7-9/11) sediment traps moorings were deployed in the GPSC to collect the time-series data of sinking particle and related dynamic parameters. Parameters of discrete sediment analysis were used to build continuous time-series data by interpolation, and time series analysis applied to understand the change of physical and geochemical character and their correlation with dynamic parameters. The results showed that sinking particles of different grain-size classes confront different forces in the canyon and their grain-size distribution structures are influenced accordingly. Vertical component of the flow has more influences on coarse particles, while the along canyon flow component has more influences on fine particles. The influence of semidiurnal tide on sinking particle is not clearly resoloved, but spring tide and neap tide affect them significantly. GPSC is normally a stable deposition environment dominated by tidal currents. Particle-reactive materials vary upon with clay concentration, coarse paericles vary upon with the flow field, and the change of benthic nepheloid layer thickness during spring and neap tide cycle affects the vertical distribution of particle size-groups near the bottom of canyon. The particle in the upper (rim) and lower (near the bottom) canyon belong to different transport and dynamic regimes. The upper part was affected by upwelling and shelf processes, while the lower part was affected by tidal currents. In case of episodic event, if surge-like turbidity flows pass near the canyon floor, in the waxing phase, the sinking particle would be affected by the strong momentum of resuspension and mixing which leads to a dramatic change of geochemical character of these particles. In turbidity current event, coarse sand and silt are the major particle sizes with low clay content, suspended sediment concentration about 4.41 g / l. The fluctuation of time series analysis by HHT found a frequency between 2.1~9.8 clcle per day. In the waning phase, dynamics and geochemical character of sinking particle will gradually return to those variations in tidal dominance. In winter, most sinking particles in GPSC are the source material (particles of biological origin) coming from the off-sea with the upcanyon flow during spring tide period. In summer, most sinking particles in GPSC are the terrigenous material (higher organic matter) output from the Gaoping River during typhoons, and flowing to the South China Sea along the canyon with turbidity flow.