Messing with rivers

I was wondering if it is possible to change the ending types of a river. I see in the xml that they have a cliff ending and a beach ending, is there a way to have a plains ending for the river. I wouldn't mind just designing a small lake that the river flows into.

Basically if the river ends on the plains the river changes from a cliff waterfall to a lake. Specifically I know that Heavenfall has a storm river mod, so I wonder if this is possible and what needs to happen to get this to happen?