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Pre-processing does not identify main stream channels in stream shapefile


I'm having some difficulty finding an appropriate value for "threshold flow accumulation" for the pre-processing module. I started with large values (10000, 3000, 1500, 1000) and found that my main stream channels were not being identified. (See picture below "stream1000".) I am comparing to an actual map of my streams. 

Logically, a lower value would facilitate identification of more stream beds. However, even lower values are failing to identify main channels, despite identifying hundreds of nonexistent branches. (See picture below "stream100".) Note that for the upper stream, the model does not identify a continuous channel, and for the lower stream, only small parts of it are identified.

Note: blue = actual stream channels
          green = predicted by toolbox
          red = catchment of interest

As always, thanks for your ideas!
James (Ethiopia)

1075 x 670 - 2M
1103 x 698 - 2M


  • I might add that I'm using a constructed DEM created using both contours and streams.  So, it doesn't quite make sense why streams are not appearing.
  • RichRich Administrator, NatCap Staff
    Hi James, I can't be sure without your data, and the pictures didn't post, but is it possible your DEM has pits in it?  It's likely the problem if your stream network is showing many disconnected streams across the landscape.
  • swolnyswolny Member, NatCap Staff
    Hi James -

    When you say that you're using a 'constructed DEM', what exactly does that mean? What did you start with, and what process did you go through to create the DEM that you're using in the pre-processor?

    I can see your images, and it really does look like you've somehow got a lot of pits/sinks in the DEM that cause the result to be so discontinuous - have you tried doing a Fill or two?

    ~ Stacie
  • Thanks Rich and Stacie,

    I changed two things based on your suggestions and it now works:

    - the DEM had been constructed in "Topo to Raster" in ArcGIS using spot heights, contours, and stream shapefiles derived from a topography map. I then noticed that the rivers were heavily indented (unrealistically) in the DEM. I have now remade the DEM, this time excluding the streams, and this resulted in a normal-looking DEM.

    - I had also forgotten to fill the DEM (rookie mistake!)

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