No, I would caution against that. I think it depends on the location of the river and the uses of the river. You could have a larger river in a relatively remote location that doesn't have the same sort of risks attached to it from commercial activity.
What you need to do is look at the critical importance of the river, its recreational value, its commercial value, and then determine at that point whether and where navigation fits in. I think that would be a better approach to take than trying to do it on size or length or whatever.