As can be seen from my name/title <----
I'm all for this, in any implementation at all, even a bad one. Flavor and all.
Silveron - I want a spillway too..being able to build a tile that could be set like a combo pressure plate/floodgate that allows water above level x to pass through would make all sorts of interesting water works. However a universal 7/7 depth for every tile would be a memory hog I am sure.
The whole flooding thing could be solved with the physics already on hand. Anyone that has played with pressurized water knows that a lot of water can move very far and quickly if it has 10 Zlevels on water column pushing it. So...
When the game generates the river, it (as suggested) tries to make a floodplain around the river, depending on type of river and local terrain. IE a brook would have a narrow one, and the floodplain would be narrower in granite than in silt.
When it comes time for the flood, the game just generates an huge volume of water, under pressure, on the tiles one z-level above the current source tiles. It will cascade down the river as well as move into the floodplain in a somewhat chaotic manner.
When I get a chance (after I finish the Goblin Rail Gun) I'll use my current fort to attempt to simulate this, and report if it works at all. I do have about 10000 units of power I am not using....