I would suspect the barriers things (including the tallcorn) don't cross ARE the canals! This was "natural traverses" for the feature, yes?
(As an aside: Checking back on the original post, I totally failed to put in that in a full-scale playtest, this step comes before adding people at all; emergence of peoples, culture formation, and history get entire, multiple rounds unto themselves. So if we kept this world to take it into the future rounds, the tribes you noted in wouldn't roll onward, which is a shame, as they fit in very nicely.)
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The tribe bit actually kind of says to me that I need to put in a specific note at this stage on "How to note emergent features", so you'd be specifically prompted to say things like "The canals all radiate from a the center of the grassland, a creating a natural gathering site", with the subtext of "Let's have this become a hub for a culture later, guys!".