Dagu River Channel Extraction and Change Detection Based on Landsat Time Series

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The objective of this study is to analyze the Dagu River channel migration and its change causes using multi-temporal remote sensing data since 1973 to 2016. The research was based on six periods of wet season’s cloud free Landsat images which are Landsat MSS for 1973、1980, Landsat TM for 1992, Landsat ETM+ for 2002、2010 and Landsat OLI/TIRS for 2016.Ratio method and Modified Normalized Differences Water Index (MNDWI) method were used to extract the channel information because they can avoid possible shadowing by mountains and terrain differences. Human-computer interaction was then used to refine the result after the channel information were converted into vector layers in Arcgis. Furthermore, several characteristic factors were applied to describe the migration of river channel based on GIS layers. They include river’s length, average width, river area, area change rate and river bend coefficient. Through directly comparing the changes between characteristic factors of six images, we achieved the final result of river channel migration process.The dynamic change of Dagu River channel shows different characteristics during different phases. During the period of 1973-2002, the area and the width of Dagu River generally showed an decreasing trend, although its length presented slightly change. Meanwhile, the river bend coefficient increased continuously.The river channel migration reasons are probably as follows: 1) Water supply decreased since there was an obvious increasing of annual temperature. As a result, evaporation and irrigation water increasd and the flow decreased. 2) The over-exploitation of Dagu River water resources for urban and rural water supply has led to the lowering of water level and flow reduction in Dagu River Basin. And it even presented zero flow in some river sections.For the second stage (2002-2016), the area and the average width of the river channel showed an increasing trend. The average width continuously increased from 220.41 m in 2002 to 395.35 m in 2016. The river bend coefficient is obviously reduced. The average precipitation increased, but there was a slight increasing trend. More recent changes have been attributed to improved hydrologic conditions as a result of river dredging, building sluice and dams, widening the river and curve cutoff works.The changes of the river are the reaction of natural and human factors. This analysis of research relating to the change causes of the Dagu River channel from 1973 to 2016 showed that human activities have come to be as the controlling factor.