A quick update on my progress with the terrain editor.
As you can see Ive added simple water and adjusted the terrain significantly. In the unity packages I found simple water which gives a glistening surface with some movement, my river is simply a plane using this material and its associated script. In order to create the river bed I simply moved up the field terrain around it and heightened the mountain in certain places. After this I redid all the trees to better fit this map.
Currently I am messing around with SFX, I was attempting to create a flowing river sound that would originate from the entire place but it seems this may not be possible without some kind of plug-in. Previously at a Brighton Unity Group meetup someone was explaining how to use such a plugin, although I dont remember the name.