Dwarf Canals!

[adwyn]

IMC campaign I have long had a dwarven city that relies on trade derived from a set of 25 mile long canals that go under a mountain range to connect two major rivers. In addition the dwarves operates locks along the two rivers in place of older portages.

 

[Mapleaxe]

I had a similar concept in one of my campaigns, except instead of dwarves, I used kobold traders on barges in an underdark river system.