Researches On Water And Suspended Sediment Discharge Delivered By Mountainous Rivers Of The West Taiwan To The Sea

Water and suspended sediment discharge delivered by rivers to the sea play an important role in the geological,biological and chemical processes,including river channel evolution,estuarine erosion and accumulation,and the biogeochemical cycles between the land and the ocean.Researches on the variations in water and sediment discharge can explore the hydrological processes and river regime changes,and can also improve understanding of the continental-oceanic interaction and the material circulation and migration on the earth surface.Numerous researches have been conducted on fluvial water and sediment discharge,however,most of them focused on the world's largest rivers,such as the Amazon River,the Nile River,the Mississippi River,the Yangtze River and the Yellow River,while little attention has been paid to small rivers.Small mountainous rivers in Taiwan are different from those large rivers,with short length,steep slope and high sediment yield.In this study,variations in water and sediment discharge of major rivers in Taiwan were investigated and the influence of geomorphology evolution,climate change and human activities on water and sediment changes were also discussed,based on long-term water and suspended sediment discharge from the Water Resources Agency of Taiwan.The main conclusions include:Extremely high water and sediment yield with significant seasonal variations,innter-annual changes,multiple periodicities and arupt changes.Compared with the Changjiang River,water discharge and sediment load of mountainous rivers in Taiwan are very low,but the water and seiment yields are extremely high.The highest water is delivered by Gaoping River(2.6x 106 m/(km2·yr))while the highest sediment load is delivered by Zhuoshi River(1.70×104 t/(km2·yr)).The intra-annual distribution of water and sediment is uneven,with 62%?90%discharge and 77%?98%sediment delivered in the flood season.The inter-annual changes of water and sediment are significant,flood years and dry years appear alternately.Moreover,both water and sediment discharge showed observable periodicities on the 2-year,5?8-year,12?17-year and 22?25-year time scale.Topographic condition and its evolution stage is an important factor triggering the variations of fluvial water and sediment in Taiwan.Hypsometric Integral(HI)analysis showed that the topography of Danshui River and Gaoping River are at old stages with a low sediment yield,while topography of Zhuoshui River is at maturity age with a high sediment yield.Danshui River exhibited the strongest concave hypsometric curve,suggestting the longest geomorphologic evolution time and the lgreatest erosion degree,which resulted in the lowest sediment yield.Gaoping River showed a weaker concave hypsometric curve than Danshui River,indicating a shorter evolution time and higher sediment yield.The hypsometric curve of Zhuoshui River is almost a straight line,indicating an unstable transportation process,and leads to a high erosion rate.East Asia Monsoon,Meiyu and Tropical Cyclones dominante the variations in water and sediment discharge of mountainous rivers in Taiwan.Owning to the influence of Meiyu and tropical cyclones,the water discharge and sediment load of the three rivers show peaks in June and August.Located in the East Asian Monsoon region,fluvial water and sediment load in Taiwan are also significantly impacted by the monsoon.The joint effect of monsoon and tropical cyclone causes a significant variance between flood season and dry season,resulting in significant seasonal variations of water and sediment load of major rivers in Taiwan.Besides,the trends of water discharge and sediment load in different periods may be related with the variations of East Asia Summer Monsoon and Typhoons invading Taiwan.Human activities have little impact on the variations in water and sediment of major rivers in Taiwan.Human activities,including reservoir construction and water diversion project,play an important role in trapping sediment.Danshui River has a much smaller sediment load than Zhuoshui River and Gaoping River,because of the significant sediment retaining of human projectstherefore,the water and sediment discharge of the Danshui River showed decreasing trends.By contrast,the reservoir has less influence on the variations of water and sediment load in the Zhuoshui River and Gaoping River,given that Zhuoshui River has a huge sediment load and heavy siltation in reservoirs while the Agongdian reservoir in the Gaoping River is far away from the river channel.