More than one NavArea for Landscape based on e.g. Physical Material?

Hey,

Is it possible to assign more than one nav area to a landscape?
E.g. I want some units to be able to walk on grass and some should be able to also cross rivers.

Nav Modifiers would work but I would need a lot of custom shapes and its not feasible with the amount of different areas.

I was hoping to be able to do it based on physical materials but cant find a way.

Thanks!