An evaluation of morphometric parameters of two drainage networks derived from different sources was done to determine the influence of sub-basins to flooding on the main channel in the Havran River basin (BalIkesir-Turkey). Drainage networks for the sub-basins were derived from both topographic maps scaled 1:25.000 and a 10-m resolution digital elevation model (DEM) using geographic information systems (GIS). Blue lines, representing fluvial channels on the topographic maps were accepted as a drainage network, which does not depict all exterior links in the basin. The second drainage network was extracted from the DEM using minimum accumulation area threshold to include all exterior links. Morphometric parameters were applied to the two types of drainage networks at sub-basin levels. These parameters were used to assess the influence of the sub-basins on the main channel with respect to flooding. The results show that the drainage network of sub-basin 4-where a dam was constructed on its outlet to mitigate potential floods-has a lower influence morphometrically to produce probable floods on the main channel than that of sub-basins 1, 3, and 5. The construction of the dam will help reduce flooding on the main channel from sub-basin 4 but it will not prevent potential flooding from sub-basin 1, 3 and 5, which join the main channel downstream of sub-basin 4. Therefore, flood mitigation efforts should be considered in order to protect the settlement and agricultural lands on the floodplain downstream of the dam. In order to increase our understanding of flood hazards, and to determine appropriate mitigation solutions, drainage morphometry research should be included as an essential component to hydrologic studies. © 2008 Springer-Verlag.