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ZOCs do cross the rivers as long as the units are not prohibited from crossing the feature.
If the units were prohibited from crossing unless at a bridge then they would not cross.
I am paraphrasing Dean's answer to this question over on CSW.
I just saw Dean's answer too.
Dean from CSW wrote:
No exception there, both features can be crossed by the units...since they are not P terrain, they do not inhibit ZOCs.
(Note that the illustration in the rules makes a point that the river there can only be crossed at the bridge.)