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Taiwan's Rivers Have a High Sinuosity of Meandering, with the Upper Stream of Lao-Nung RiverTopping the List—NTU Department of Geoscience's Research on Sinuosity of Rivers Meandering in North Pacific Published in "Science"

On March 18th, the internationally renowned journal "Science" published the results of a joint research by National Taiwan University, the Columbia University of the U.S., the Tokyo University of Japan, and the Cambridge University of Great Britain etc. The research found that, as Taiwan was subject to heavy rainfalls and flood disasters, Taiwan's rivers have a very high sinuosity of meandering among the islands in North Pacific region, with an average value of 1.4, second only to the Luzon Island of the Philippines. And the Zuo-Kou River, situated at the upper reach of the Lao-Nung River, has a sinuosity in excess of 2.0, which is most certainly the river with the highest sinuosity.

Professor Hongey Chen of the Department of Geosciences points out that, this international research project was geared toward studying the phenomenon of meandering change in the rivers on Japan, Taiwan, Philippines, Borneo, New Guinea (all in the North Pacific Region) to find evidence of the role that climate or geological environment played in the sinuosity of the rivers. The research primarily used a meandering degree grid, monthly average rainfall, and the probability density function of typhoons, to study the relationship between the rivers sinuosity and the climate.

The results of the research indicates that, the changes in the rivers sinuosity basically are in direct proportion to the amount of rainfall and the intensity of the floods. Because the frequency of monsoon, typhoon and the intensity of flood are high, Taiwan, Luzon (in Northern Philippines) and the Kyushu Island of Japan (in Southern Japan) are areas with a high sinuosity of meandering rivers. Around the Equator and middle latitude regions, where there are fewer storms and fewer floods, the sinuosity of the rivers are also less.

The results of this research tells us that, although the North Pacific Region is affected by the tectonic plates, but the influence of crustal uplift rate on the sinuosity of rivers meandering is less than that of the climate. For instance, the crustal uplift rate of Luzon island is -1mm per year, relatively lower than that of Taiwan (-6mm per year), but, because the frequency of typhoon and the intensity of floods in that area are higher than those in Taiwan, therefore Luzon Island has a higher degree of river meandering, averaging 1.5. Taiwan has the second highest sinuosity of river meandering, averaging 1.4. However, the Zou-Kou River in the upper stream of the Lao-Nung River in Southern Taiwan, owing to the intensity of the flood and to the broken conditions of the rock layers, has a sinuosity in excess of 2.0, which is the highest in North Pacific Region. Basically, the rivers with high sinuosity of meandering are spread between 8 and 25 degrees of latitude. This result shows the importance of climate, since the atmospheric circulation in this area is associated with the steering effect, and has a close corresponding relationship with the frequency of typhoon strike.

Professor Hongey Chen, who has studied the geological features of Taiwan for a long time, points out that, in addition to climatic factors, susceptible geological conditions such as the rock strength, the fracture of the rock layers are also major factors influencing the meandering of the rivers. When the riverbed is widened or increases sinuosity curvatures, the rate of lateral erosion can be increased more than 10 times than the downward erosion shear stress. And, with the passing of times, this phenomenon will only increase its lateral erosion, resulting in the sinuosity and expansion of the river beds, the formation of river terraces, the asymmetry of the valleys, and even meandering of the rivers.

Professor Chen specifically points out that, the rocks strength in Southern Taiwan averages under 25 Mpa, belonging to the ranking of weak rocks, plus the fact that the discontinuities extends to many places in central and Southern Taiwan, making the rock layers appear to be fractured. And the higher frequency of rainfall and the delay of the rainfall make the both sides of the river bank and its adjoining slopes more unstable, and make the lateral erosion become harder. Thus, flood frequency, flood shear, and the sediments will have corresponding increase on the erosion of the river banks and river beds. These phenomena of natural change can find evidence in landslides, debris flow, or erosions caused by change in catchment morphology, and are most explicit in Taiwan and the Luzon Island of Northern Philippine.

The published paper was the result of international research cooperation among National Taiwan University, the Columbia University of U.S., the Tokyo University of Japan, and the Cambridge University of UK. It also received support and funding from the National Aeronautics and Space Administration(NASA) of United States, the National Science Foundation of U.S., the National Science Council of Taiwan, and the Water Resources Agency of the Ministry of Economic Affairs. Aside from Dr. Stark of Columbia University and Profess Hongey Chen of National Taiwan University, other major participants in the research include Dr. Barbour of Columbia University, Professor Hovious of Cambridge University, Professor Hayakawa of Tokyo University, Professor Hittanji and Professor Xu of Sapporo University, Professor Fukahata of Kyoto University, Professor Ching-Weei Lin of National Cheng-Kung University, and Dr. Ming-Jame Horng of the Water Resources Agency.

Starting from 2001, the NTU Geosciences research team has been cooperating with the research team of the Columbia University of the U.S. for more than a decade. Their research achievements have been published in internationally renowned journals such as "Nature", "Geology", "Geophysical Research Letters", etc. This present study took almost three years to complete its analysis, and can be provided as an important reference for people who are interested in studying the islands in the North Pacific Region, and in studying Taiwan in the natural evolutionary process.

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