I have a strange behaviour in QGIS with GRASS tools. I am trying to create basins in the alp region and use he SRTM3 data. As I can see on the flow direction raster the SRTM3 data seems to act nice: flow direction with basin overlay I was using the r.watershed algorithm with a minimum size of 1000, 2500 or 5000 and I am always seeing the same problem: straight lines across a slope and not following the main flow direction raster (visible in the center of the above screenshot).

Can anyone give me some advice on how to take care of this issue?

I was using QGIS 2.12.3, Ubuntu OS, the SRTM3 39_03 dataset and lots of coffee. I tried D8 and non D8 algorithm as well as different values for RAM usage... I will try it once more with an older version of QGIS, but it seems crazy!

  • Can you calculate the aspect map instead of the hill shade and post it here?
    – sig_renato
    Commented Feb 8, 2016 at 3:29
  • @sig_renato this is not the hillshade up there. It is the Flow_direction!
    – Riccardo
    Commented Feb 8, 2016 at 7:12
  • Did you fill in the sinks in the DEM before using it? Commented Feb 8, 2016 at 7:18
  • 1
    as it was explained here, it is not necessary: gis.stackexchange.com/a/83810/18562
    – Riccardo
    Commented Feb 8, 2016 at 7:27

1 Answer 1


The problem was the parameter "allow only horizontal and vertical flow of water". I really don't know why this is set by default in QGIS/Processing!!! damn noob error! better basins!!!


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