Nuttz...before you go totally nuts, keep in mind that the sub-tile type for rivers must match in a 1 mile sector. In other words if you manually assign sub-tiles they must all be type 1, 2, 3 or 4. So a 1 mile grid sector would be like 1,1,1,1. You can't mix 1,2,1,2 etc.
That is of course unless you want to re-define the river objects too.