Abstract: The automatic detection of drainage networks from digital elevation the models(DEM)is affected by flat areas and closed depressions.A variety of methods have been proposed to treat these two features,but the flat areas and closed depressions are processed only after they have been created,so as to generate many spurious and parallel drainage courses.a new algorithm for treating the flat areas and closed depressions is proposed in the paper.It is known that the flat areas and closed depressions often arisen because of low quality input data,interpolation errors during the DEM generation et al.Therefore,they can be removed by adding elevation information to input topographic data. The proposed approach can lead to better match between observed and modeled flow structure and produce more realistic results in application.