Results and discussion A study of suspended sediment variability under the conbined influence of tide and river discharge at the mouth of a small mountainous river A study of suspended sediment variability under the conbined influence of tide and river discharge at the mouth of a small mountainous river Rick J. Yang, James T. Liu, and Kevin J. Chang Institute of Marine Geology and Chemistry, National Sun Yat-sen University, Kaohsiung, Taiwan 80424 Introduction 　Globally, sediments for most coastal depositional systems are supplied by rivers. Hydrodynamics influence the sediments in their transport and distribution. The aim of this study is to understand how the hydrodynamic around the river mouth affect the suspended sediment population in the river plume. Materials and methods 　The Gaoping River mouth is a shallow and turbid estuary (Fig. 1). Field monitoring and sampling were carried out to study the hydrodynamic and sediment dynamics in the estuary and the river plume. In the estuary, an instrumented tetrapod was deployed with an upward-looking ADCP, two SCTDs, and one LISST-100s (Laser In-Situ Spectral Transmissometry) between May 22 and May 25, 2005 (Fig. 2). Off the river mouth, we collected data by using CTD, sb-ADCP and LISST-100 on board R/V Ocean Researcher III between August 27 and August 28, 2006 (Fig. 5 and Fig. 6). Fig 1. The map of the study area Longitude (deg. E) Latitude (deg. N) Gaoping Submarine Canyon River Tetrapod OR-3 Fig. 3 Volume concentration in the Estuary Fig. 2 Salinity and current in the Estuary 材料 Results and discussion 　　In the estuary, the observed currents are dominated by semi-diurnal tides (Fig. 2). The Gaoping river plume moves according to the tidal current. The volume concentration of thirty-two suspended sediment grain sizes were observed by the LISST-100, so we can compare the volume concentration to the salinity and the current. According to the correlation analysis (Fig. 4), the salinity is the most important factor that influence coarse-grained(>63μm) sediment and there is no time lag. The salinity is also an important factor to influence 3-10μm and 10-63μm grain sizes, but they have about one hour time lag. The current is an important factor for influence all grain sizes, but they also have about one hour time lag. That means river-sea water exchange is the greatest process controlling the suspended sediment movements. On the other hand, the plume largely contains the grain-size of >63μm. The time lag to show that current causes resuspension of all grain sizes. Off the river mouth, we found correlation between low salinity and the higher volume concentration of coarse-grained sediment(>63μm) (Fig. 5 and Fig. 6). Fig. 5 Time variability of volume concentration profiles off the river mouth Salinity Depth (m) Volume concentration (μl/l) 13:00 14:30 15:00 17:00 19:00 20:00 21:00 22:00 01:00 02:00 03:00 04:00 05:00 06:00 07:30 08:00 09:00 11:00 12:30 13:30 14:00 8/28 8/27 >63μm 3~10μm 10~63μm Fig. 4 Correlation analysis results Conclusion 　　Coarse particles (larger than 63μm) transported from the Gaoping Estuary to the sea are mainly dispensed by the plume. And the factors influence the distribution of fine size particle (3~10 micrometer) are resuspension or the marine source. Fig. 6 Time variability of salinity and transmission profile off the river mouth END Acknowledgement This study was funded by the National Science Council of Taiwan under grant numbers NSC95-2611-M-110-018 and NSC96-2611-M-110-011.