Characteristics of evolution and its impact factors of the Yellow River along the Lanzhou urban reach from 1949 to 2005.

with the help of GIS technology,the channel information of the Yellow River in Lanzhou urban reach were taken from the actual land map in 1949,1975,1980,2000 and 2005.Fistly,the river evolution characteristics were analyzed according to the scale contraction intensity index,such as the channel scale,river islands scale, the main riverbank length,average river width,river islands density and river form index,and then the impact factors of river evolution were discussed in the light of urban population and urban area,urban flood control embankment and strand river road in deferent years.The research results indicated as follows:(1) The Yellow River was in a negative contraction process for the river channel exploitting from 1949 to 2005,whose percentage in Lanzhou urban basin area was decreased from 22.3%in 1949 to 4.6%in 2005 of the basin space in Lanzhou urban because large river surface and many river islands were exploited,and because many anastomosing branches were cut and infilled and died away.At the same time,the main channel was shifted toward north and the river bank turned winding.The process of river channel contraction was fluctuant just likeU-shape,and its constraction from 1949 to 1975 was the strongest and from 1975 to 1980 was the slowest.All these led up to the evolution of the river channel patterns changing from braided channal to bifurcated one,and to meandered one.(2) The urban expansion was an interior driving factor that 1%of the urban area expanding would contract 0.4%of channel area.Urban construction of flood-controlling embankment and riverside road were the direct forces,which could satisfy the urban flood control demands and trafic demands.However,much land by the river channel was surrounded to satisfy urban development.City plannings and system-policies were the regulation factors that could change the exploiting mode,its time sequence and intensity.Store water and low water started from 1969 were the externally inducing factors because the runoff was decreased greatly and distributed balancely among the yeas,which led to many river islands being out of the river surface and connecting each other.In all,the river evolution was driven under the joint effects of the multi-factors.