Deriving Digital Elevation Models (DEMs) from topographic map data is essentially an interpolation process. So, when a DEM mosaic is obtained from a set of DEMs, elevation errors arise, especially along common boundaries. To solve this problem, DEMs are derived so that they have areas in common with their neighbours, and the elevations of the cells in the common areas, i.e. overlapping areas, are recalculated during mosaicking. A DEM mosaic obtained in this way will be different from the individual DEMs, and the difference will depend on the overlapping distance. In this study, in order to determine the effects of DEM overlapping distance on hydrological analysis, drainage networks and basin boundaries are derived by hydrological analyses on DEM mosaics obtained from 0, 1000, 2000, 3000, 4000, 5000 and 6000 m overlapping DEMs derived from 1:25,000 scale topographic map data, furthermore, horizontal and vertical positional errors of the basins, various geomorphometric and topographic parameters are calculated. Consequently, it can be said that the overlapping distance affects the geometries of the derived drainage networks and basin boundaries and, accordingly, the horizontal positional accuracies, and geomorphometric and topographic parameters of the basins, but do not affect the vertical positional accuracies.