Finding Baum's idea of Oz is a matter of digging. When I was a kid I read ALL of the Oz books. And one book had a MAP of Oz. It showed a shaped map of Minnesota darkened to show two rivers converging diagonally on the left and right. And that was as described in the geography of Oz in one of the Oz books. For that specifically, you will have to dig this out.

But I DO remember the shape of the Minnesota map and the two river in Minnesota that flow down southerly.