When I create stream network in ArcGIS, for example, if there is a cell of more than 100 flowing into that, it assumes stream. Does ArcGIS decide to cells more than 100 or 80? If I decide, what are the effective parameters ?


I wrote a detailed answer to another recent question regarding extraction of streams from DEMs here. However, the gist of it is that when you extract a stream from a flow accumulation raster (upslope area) you are effectively saying that there exists a geomorphic threshold that relates the amount of discharge (upslope area is used as a surrogate for discharge) needed to maintain a steam channel with the continual removal of in-channel sediment and the forces acting to fill the channel in (mainly sediment delivery from bank collapse and overland wash). So we're saying that there exists one critical upslope area value in the landscape which will define the location of channel heads. In reality of course, this is an oversimplification that doesn't account for varying geology, vegetation, soils, macro-pore and pipe structures, slope gradient, and other things that act to vary the position of the channel heads in a stream network.

So, how can you derive this critical threshold in flow accumulation? The typical way to do it is to examine a vector stream network derived from the blue lines on a topographical map or from aerial photography. Locate the channel heads in the network and then try to find the average flow accumulation (upslope area) value that corresponds to these landscape positions, realizing that given what I said above there isn't going to be a perfect single value but rather a range. There are also alternative models that use thresholds in the combination of upslope area and slope gradient (e.g. the stream extent will move towards the ridge if the local slope gradient is greater).

Once you've decided on your critical threshold value, based on interpreting channel head data, you simply need to apply the threshold. The Con tool in ArcGIS works for this but so too does a simply > (Greater Than) operation in the Raster Calculator of most GIS. Once you've done this, you may want to do further processing to remove exterior links in the stream network that are shorter than some specified threshold, e.g. no stream links shorter than two grid cells. There are tools available to do this type of stream network analysis, though I'm not sure what support ArcGIS offers for this.

  • I decided avarage flow accumulation values for example 150. And then, I enter this value at the Con tool. Is it right ? – zeynep Oct 1 '14 at 11:14
  • @zeynep Yes that should work for you. – WhiteboxDev Oct 1 '14 at 11:44
  • @zeynep I'm glad to help. – WhiteboxDev Oct 1 '14 at 12:46

It sounds like you want to come up with your own threshold for determining streams based on the Flow Accumulation Values. You can do this using the Raster Calculator in Spatial Analyst (under Map Algebra in 10.x versions). You would use a Con Statement to create the streams like this (using 200 as an example threshold) ...

Con("FAC" > 200, 1, 0)

If you want NoData instead of a zero value, just leave the second argument out entirely ...

Con("FAC" > 200, 1)
  • You can apparently use Set Null as an alternative method but Con works well for me. – jbchurchill Sep 25 '14 at 15:40

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