I'm trying to complete a school project to build a GIS system, and my group has chosen to try mapping flood plains and we're running in to issues. We've downloaded a DEM off a scholar portal and imported it fine, combined the tiles in to a single DEM raster layer and we thought that by using the Flow Direction tool in Flow Accumulation after that to generate places where water could accumulate. In doing this however, we get a messy looking output file for flow accumulation that looks like little lightning bolts all across the map. The Flow Direction tool looks to have outputted our data OK, as it looks similar to a topographical map in a way. I'd have expected that we'd see some kind of solid areas in the flow accumulation output notably near rivers and lakes and such. Can anyone provide any input in to this and any suggestions as to how we could fix it? It would be much appreciated. Thanks in advance!