D&D is a Role Playing Game. Characters play a role in a story. If this is cnsidered to be a point of rule ambiguity, ask whether the situation tells a good story. If so, go with it.
To me, a tortle druid screaming at the river that it shall not take his friends as it batters and throws him against the rocks is a perfectly fine use of rage. However, I would not consider this an ambiguous situation. As long as there is damage he is taking that is actually a threat to him, or he is fighting against something, I'd allow it. I have also been known to allow it in other situations, such as when the barbarian was in a weight lifting competition and had a huge crowd cheering him on.