The Study Of Changes In Areas Of River Islands In The Mainstream Yangtze River And Lower Yellow River,and Its Controlling Factors

As the two largest rivers in China,the Yangtze River and the Yellow River play a vital role in both transportation and interregional resource allocation.There are many river islands in the mainstream of the Yangtze River and the lower Yellow River,and their dynamics are impacted by runoff,upstream sediment load and human activities.With the construction of cascade reservoirs and water conservancy projects,the runoff and sediment environment downward have changed significantly.This also changed source materials for river island development.The dynamics of river islands inevitably has a significant influence on environmental evolution,human activities and economic development within its watershed.Therefore,it is of great significance to perform research on the changes of the river islands in the mainstream of the Yangtze River and the lower Yellow River.This work can provide reference for monitoring of long-term changes of the area of river islands in the Yangtze River and the lower Yellow River,as well as the management of water conservation projects in the Yangtze River and the Yellow River basins.The present study is based on remote satellite images of river islands in the Yangtze River and Yellow River from 1986-2019 provided by the United States Geological Survey(USGS).In combination with the average annual runoff and sediment discharge data over the past?40 years,we reconstructed the changes in river islands and examined its possible controlling factors.Main findings of this work are specified as follows:1.River islands in five sections(A-E)of the Yangtze River along the direction of estuary to Yibin in the upper upstream show overall patterns of increase,decrease,increase,decrease and decrease,respectively.Compared with the reference period(1986),the change rates of island areas in each section are 6.46%,-0.87%,0.76%,-0.15%,and-0.26%,respectively.River islands in the upper stream(section E)show an slight enlargement.In the middle and lower parts(A-D sections),some river islands increase its area,but some others shrank.2.In terms of spatial change,the Hu-An section(from estuary Shanghai to Anqing,Anhui Province)and Huang-Hong section(Huangshi,Hubei Province to Honghu City)of the Yangtze River show the most evident increase in river island area.In contrast,the An-Huang section(Anqing,Anhui Province to Huangshi,Hubei Province)and Hong-Yi section(Honghu,Hubei Province to Yibin,Sichuan Province)show an overall decrease in areas of river islands.The most significant expansion of river islands appeared in Hu-An section,and the most evident shrank occurred in Hong-Yi section.The river islands were divided into three types,i.e.large,medium and small,based on their area.In general,all these three island groups show very similar trends.The Yangtze River has the largest number of small(1-10 km~2)and medium(10-50 km~2)river islands in its mainstream,and the change rates of their areas are high in the past?40 years.The number of river islands with area?50 km~2 is small and the changes in their areas are minimal among the three island groups.3.The average annual runoff of the Yangtze river did not change significantly since1980.However,influenced by the construction of the cascade reservoir and the soil and water conservation project in the upper and middle parts of the Yangtze River("Changzhi"project for short),the sediment transport has been gradually reduced since90s.The areas of river islands in the upstream of the Yangtze River decreased.It is identified that sediment discharge is the main controlling factor of the island area change.Under the influence of the impoundment of the Three Gorges Reservoir and the construction of"Five Rivers"of Poyang Lake on the water and sediment regulation in the middle and lower reaches of the Yangtze River,the sediment load decreased obviously,but the areas of river islands in the middle and lower parts of the Yangtze River began to increase gradually,implying that the impacts of the dam construction and Poyang Lake have decreasing influence on river island development toward the estuary.The further from the dams,the more natural the developments of river islands in the Yangtze River are.4.Both areas and number of river islands in the lower Yellow River decreased during the period of 1986-2020(the absolute decrease in total area is more than 55.8 km~2).The periods of 1986-1995 and 2005-2010 show the most significant decrease,and the periods of 1995-2000 and 2005-2010 were found to be transition time.In terms of spatial change,the areas and number of river islands in the Yellow River near the Xiaolangdi Dam(from Taohuayu Village in Zhengzhou City,Henan Province to Jiahetan Village in Lankao County,Henan Province)show the most evident decrease,followed by the Jia-Ai section(from Jiahetan Village in Lankao County,Henan Province to Aishan in Dong'e County,Shandong Province).The section below Aishan near the estuary(from Aishan to the estuary in Dong'e County,Shandong Province)show the smallest change.5.Most of the river islands in the lower Yellow River fall within the two area groups:0.1-0.5km~2 and 0.01-0.05km~2,and they also showed the most significant change.In contrast,the number of river islands with areas>3 km~2 is the smallest and only a s small part of them shows a decrease in area.As a result of soil and water conservation projects in the middle watershed of the Yellow River,the average sediment transport during the period of 2000-2015 has been reduced by about 86.8%compared with that prior to impoundment(1986-1999),and the reduction in sediment discharge has also had a significant impact on the river islands.Before the year 2000,the number of river islands in Ai-Jia section shows a continuous decrease,and the number of peach cracker beaches first decreased and then increased.For Ai-Jia section,the number of islands increased first,but then decreased.After 2000,the number of river islands in the lower section of Ai-Shan section decreased gradually,and disappeared completely at 2010.