Automatic extraction of regional water resource information from digital elevation data can promote the efficiency of regional water resource prospecting and assessment in GIS. Therefore

a practical method for automatic extraction of natural river system from DEM is introduced in this paper. The method

first

needs to pre process DEM data in order to transform depressions and flat areas in the DEM into extended parts of the corresponding slopes. And then the flow direction of every grid point in the DEM is determined according to the theory of which water always flow along the steepest down slope direction. Third

the upstream catchment's area of every grid point can be calculated on the basis of the data of grid point flow direction. Forth

giving a threshlod value of upstream catchment's area picks out the grid points in river lines. Finally

the river system is pursued from DEM according to the flow direction data of grid points in river lines. The method has been applied to extracting the natural river system automatically from the DEM of a testing area. The natural river system extracted automatically with the method is basically consistent with that drew manually based on the corresponding topographical map. This experimental result has proved that the automatic extracting method is effective.