What would you think of a canal linking the God’s Eye to the point where the Trident branches out? This would link all rivers in the Riverlands (thereby unifying the region economically), and would also allow the entire Riverlands to transport grain down the Blackwater Rush to King’s Landing similar to the Mander-Blackwater canal you proposed (as the Riverlands are the largest producer of grain behind the Reach). Does this sound like a good idea to you?