River islands' change and impacting factors in the lower reaches of the Yangtze River based on remote sensing

摘要

There are many developing river islands in the Mawutong section of the Yangtze River, and their sizes change rapidly. Changes in eight river islands were studied using Multi-Spectral Scanner (MSS in 1979) and Thematic Maps (TM in 1990, 2001, 2006 and 2009). The spectrum of typical objects in river islands was analyzed by ISI921VF-256T Field Spectroradiometer, bands combined, and images interpreted by geometric correction, subset images, binarization processing, convolution, clump procedures, and visual image interpretation. A number of factors that were considered to influence the evolution of river islands were analyzed. They include annual runoff, annual sediment, and soil particle composition of river islands. The results show that: 1) the ISI921VF-256T Field Spectroradiometer can help to identify the correct spectrum of typical objects in river islands, hence the selected bands (three visible and one near infrared) can be used to extract the variation of river islands' area based on digital image processing and visual image interpretation; 2) in general, the eight river islands lengthened in 1979-2006 at various rates, with the highest rate during 2001-2006. Their sizes decreased rapidly in 2006-2009 except for two river islands; 3) erosion and sediment deposition are two major physical processes that influence the evolution of river islands. As no significant trend of annual total runoff in 1953-2010 was detected, it is likely that the construction of large reservoirs upstream and hence the runoff dynamics and sediment loads in this region have more impact on the changes of river islands than do climate changes.