I got the idea from the actual dam on the Yangzi River, but I make no claims that it is a factual rendering of the place in China. :)
Basically its a a large map with three rivers at the top, middle and bottom of the map. The two end rivers cut through cliff face (more gorges), while the middle river is "damed" ending at the face of the cliffs.
To represent the dam, I raised the eleveation 2 levels, and then where I wanted the river to appear I lowered the new Elevation by one, creating the river bed, filled that with river tiles up to the edge of the cliffs. And then placed a double row of rocks where the river ends and then another double row of rocks at the base.
There are pigs and alligators, but the alligators need to be removed or placed somewhere else because they will eat all the new immigrants... :/