On Metaphors

What is it with, "like a tree that stands by the waterside, we shall not be moved"?
When I think of the waterside, I tend to think of the edge of a river, and when I think of a river, I imagine a thing that periodically rises up to overwhelm the countryside, sweeping away everything along its banks. So the waterside seems like the worst place to be if you don't want to be moved. I guess the song might suppose that humans, rather than nature, are doing the tree-removal, in which case it would make sense to keep the trees near the edge of the water to stabilise the banks. But it does seem to be a rather peculiar and dubiously-applicable metaphor.

Similarly, there is a hymn (which none of you might ever have heard) that says: "Peace is flowing like a river, flowing out for you and me, flowing out into the desert..." Now, having grown up somewhere where there actually were rivers flowing out into a desert, my experience is that they either: (1) Don't flow at all; or, (2) Flow in a seething mass of brown rapids, sweeping away unlucky motor vehicles and the aforementioned waterside trees. Thus, a river flowing out into the desert would seem to be a good metaphor for the pre-peace injustice-->justice transition, with much hanging of collaborators from lampposts and tracer bullets lighting up the sky over the besieged Ministry of Information, but is hardly very appropriate for peace per se...