ArcGIS 10.3 provides a hydrology tool "flow length" which calculates the length of flow paths within a basin to the outlet of the basin. I am trying to calculate flow path length and flow path gradient from a given pixel of interest to where it enters the stream network. Using TauDEM (which I do not have access to), they define this as the 'distance from a pixel of interest to the stream channel using the D8 flowpath direction'.

I have a 30m DEM, flow direction raster and river network. From these flow path values I need to calculate mean flowpath length and mean flowpath gradient for a catchment for use in a catchment classification study.

Using ArcGIS 10.3 for Desktop.

  • 3
    Convert cells under stream into NO DATA value, recalculate flow length
    – FelixIP
    Commented Feb 23, 2016 at 22:10
  • How would I convert the cells under the stream layer to NO DATA? I can't just reclassify the flow direction layer because the stream network isn't consistently represented by one direction.
    – Brun
    Commented Feb 24, 2016 at 16:51

1 Answer 1


I solved this problem by converting the cells under the stream network to 'NO DATA' as per FelixIP's comment and recalculating the flow length.

I set the stream network cells to NO DATA using the Raster Calc equation: SetNull(~(IsNull("Stream Raster")), "Original Flow Direction Raster")

Also of note, when calculating flow length, must set as DOWNSTREAM flow length.

  • @FelixIP I have 15 catchments and I want to do the same as what Brun has done for each catchment. Is it possible to do that for all 15 catchments at the same time?
    – shiny
    Commented Oct 16, 2017 at 7:15
  • Yes it is. What stops you?
    – FelixIP
    Commented Oct 16, 2017 at 8:24

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