Not a problem of the scenery, check the screenshot below, the trees runs correctly alongside the river, and the fence is very well on the inside.
It seems that you are using another vector terrain product (like Ultimate Terrain Europe or similar) which is conflicting, so you are seeing the river coming from that one instead.
Be sure the Scenery Areas for Fly The Lakes (airport and terrain) are above that product layer, which is where the installer puts them