We developed a global basin layer using the Srtm30 Digital Elevation Model at a 30 arc-second resolution (Cite). Hydrologic modeling was carried out in a GIS to determine the flow direction using the D8 algorithm. Basin boundaries derived from this.
Because of data inaccuracies, artifacts of the D8 model and the scale of the DEM, manual correction was required. The basins were inspected at a scale of 1:1,000,000 and, using ancillary data such as Landsat, GTN and Vmap (CITE), the DEM was adjusted by altering the elevation values to achieve the proper flow direction, definition of non-draining inland basins and a contiguous coastline.
Once a proper flow map was developed for each continent, the basins were deliniated. Basins with less than 12 square kilometers area were merged into adjacent basins. Finally the maximum flow accumulation for each basin was determined, its location used as the river's pour point (the point at which the river interacts with the marine environment).