I need to define the stream network for a huge area and I???ve finally managed to do so with the ArcGIS 10 hydrologic functions. But the results in a very small area are not accurate enough for me to reproduce the process to the whole project area (ArcGIS would lead me to deal with 1 culvert as if it was a set of many wrongly placed culverts; and I have more than 13,000 culverts in the project area!) I???m attaching a Word file with 3 images of a stream network (and the road network, once I???m studying roads drainage). The ArcGIS results are very close of what we could deduce (image 1) and confirm with satellite photos (image 2). Actually, what we can deduce is confirmed by a hydrography layer (image 3, planimetric information). But I need to work with the ArcGIS results. Because of that, I???m trying to realise how we could ???force??? the mathematical ArcGIS results so that they would better reproduce the real world. I???ve read in the ArcGIS help about the Flow Accumulation function that ???If no weight raster is specified, a default weight of 1 will be applied to each cell???. I couldn???t find much more about the use of a weight raster and I???d like to ask anyone who has faced similar hydrologic problems: COULD WE CONVERT A HYDROGRAPH LAYER (WITH DRAINAGE LINES) IN A WEIGHT RASTER TO USE WITH FLOW ACCUMULATION, SO THAT ITS RESULTS WOULD COINCIDE WITH THE PLANIMETRIC ONES? Any tips on how to do that? Any other ideas on how to teach ???common sense??? to the flow accumulation function? (Any hydrologist wouldn't draw 3 parallel drainage lines, like ArcGIS did in that small area, where there???s obvious only one preferential way to the water.) Thank you to anyone who would take some time to think on this problem and share his/her ideas with me! *** Sorry, the file is 4MB and the maximum allowed is 1MB. But its not essential to discuss the problem, I think.