I would like to extract the watersheds for a particular area using QGIS. However, I only have Aster 30m DEMs to work with, each tile covering one degree square. The problem is that I need to use four different tiles and the results do not all correlate where they meet. How can I resolve this, or am I doing something wrong?
Have you tried to create a mosaic out of the rasters and doing one watershed computation? This link is helpful for raster mosaics in qgis. Trying to make separate basins and putting them together probably won't work due to edge contamination.