There was, I seem to recall reading, some damage to the Walhalla line by the 1939 bushfires, but it did not result in closure at that time.
I'm not sure that the condition of the Wannon River bridge caused or contributed to the closure of the Casterton line, but I can't rule it out; I just don't know. But it was certainly one of a number of lines in the area closed on the same day.
Again, I don't know that the condition of the Wimmera River bridge (as distinct from the cost of maintaining such bridges) contributed directly to the closure of the Carpolac line - I think it was just a case of unfair competition from road transport and that governments of either colour didn't like rural branch lines.
So the answer isn't either the Wannon River of Wimmera River bridge.
No, the bridge of which I am thinking contributed directly to the cessation of services - and in rather a dramatic fashion.