International river basins are characterized by their wide extent where mapping earth surface features and drawing contours by topographic team become–somehow- impossible because the cost and efforts consumed to execute it become very high and may exceed reasonable limits. Here it becomes necessary to use digital elevation models (DEM) inferred by specialized scientific organizations using remote sensing. There are several DEMs available on the internet and downloadable for free. The primary factor in defining the models efficiency in building hydrological models not the least cost but the maximum reliable results and better resolution that adequate to the capabilities of the PCs. In this study, three DEMs were used to derive and build hydrological models for the Euphrates-Tigris basin using Geographic information system techniques. The resulted boundaries of Euphrates-Tigris basin were compared with three boundaries implemented by international research organizations (UNEP, ESCWA, and FAO). As a conclusion of this comparison, the SRTM—3arc DEM was the most efficient model among used models. In addition, this study indicated the necessity to reevaluate Basin’s boundaries and correct the spatial distribution of proportion for basin area between the riparian countries.