Impact of rapid urbanization on river system in a river network plain

To reveal the impact of rapid urbanization on river system in a river network plain, characteristic indexes of river system, such as grid river density, development coefficient of tributary and fractal dimension, were used to evaluate the temporal and spatial changes of river system in Suzhou City from 1980s to 2010s, based on topographic map in 1: 50000 scale and remote sensing image in 30m scale. The results were as following: The length of river system was decreased by 10.55%. From view of each order river, the length of 1st order river was increased by 3.24%, while that of 2nd order river and 3rd order river was decreased by 18.87% and 11.76%, respectively. The grid river density of river system was mainly decreased across most area of this region, and the change trend between main river and tributary was opposite in space, i. e., the grid river density of main river was mostly increased, while that of tributary was mostly decreased. The development coefficient of tributary and fractal dimension of river system respectively changed from 3.36 to 2.78 and 1.70 to 1.63, which indicated that the structure and morphology of river system became simplified. The attenuation of river system became remarkable when the land urbanization rate was greater than 40%. The response to land urbanization for each order river was different. Overall, 1st order river experienced a slight increase, to enhance the capability of flood control and drainage. When the land urbanization rate was less than 40%, some 2nd order rivers were occupied by farmland, and many 3rd order rivers were dug to improve the capacity of drainage and irrigation. When the land urbanization rate was greater than 40%, a lot of tributaries were buried and converted to construction land. And the lower the order river, the more severe the attenuation was.