I am following Strahler's method for stream ordering.
Is the green colored stream 4th order, according to this method?
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I think it's difficult to determine without knowing flow directions and seeing the bigger picture.– MaryBethMar 4 '16 at 14:15
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@MaryBeth, unless there's something bizarre about the landscape, you can determine the direction of flow just by looking at the shape of the stream junctions. In this image, it's generally right-to-left.– MarkMar 4 '16 at 21:06
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From your image, I guess that the red streams (labelled "3") are flowing into the lake and the green streams (labelled "4") are flowing out.
Thus, from a topological point of view, your stream network is equivalent to (excuse my poor drawing skills):
I highlighted the "lake" node with a big blue point. You can see that you have a loop inside your stream network (the two green branches, labelled "4").
The original Horton-Strahler algorithm cannot handle such loops (see Hierarchical Ordering of Reticular Networks (Mileyko et al., 2012) or Quantifying Loopy Network Architectures (Katifori et Magnasco, 2012), for example). Therefore, you are left with two solutions:
- either apply a variant of the Horton-Strahler algorithm (for example Mileyko et al. cited above)
- or make the (purely topological) assumption that these two branches are only one branch, that is merge them:
This is a bit of a stretch, but from this point of view you could say that both green branches share indeed the same Strahler number. In the end, the results depend mostly of what you are trying to achieve and what assumptions you are ready to make.
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To compliment this answer emphasise "where the blue dot is" it is very likely to be the centroid of the lake as you have 2 inflowing 3rd order streams and 2 outflowing 4th order streams. There are of cause other network configurations that lead to the same answer.– HornbyddMar 5 '16 at 13:57