I am following Strahler's method for stream ordering.
Is the green colored stream 4th order, according to this method?
Geographic Information Systems Stack Exchange is a question and answer site for cartographers, geographers and GIS professionals. It only takes a minute to sign up.Sign up to join this community
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:
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.