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Full Idea
A river is not just an event. One would need to specify counterfactual as well as actual boundaries.
Gist of Idea
A river is not just event; it needs actual and counterfactual boundaries
Source
Timothy Williamson (Vagueness [1994], 9.3)
Book Ref
Williamson,Timothy: 'Vagueness' [Routledge 1996], p.264
A Reaction
In other words the same river can change its course a bit, but it can't head off in the opposite direction.