So I picked a nice embark with a river running across the middle, a major river about 11 tiles wide. I want to build my fort as two towers with a connecting bridge across the middle. And I wanted to get this done early, as once you reach a certain population/wealth sieges become non-trivial obstacles to above ground projects. So this means once the initial basics are set up, time to get cracking on the river project! I'm documenting the progress here to show another potentially useful river containment method.
Goal: Dam the river with the ability to control the flow after the dam is complete.
Initial Design: Build two walled in water control stations to channel water off map, draining the river. Then, build a bridge across the river bed to provide easy control.
Initial Report:
After getting farms and a dormitory dug into a hillside, construction began. I dug tunnels under the river from the temporary fort, and they surface on both sides of the river. Walls have been erected to completely enclose the tunnels from all but flying hostiles. Once this was complete, each water house had a 1x7 raising bridge built at water level, and a 1x7 drop chute built behind the bridge. The drop chute falls to the first solid stone layer, to prevent future cavern breaching from causing trees to grow in the drainage tunnels. From the drop chute, 1 tile wide tunnels were dug to the map edge and fortifications were put in place to drain water.
After these constructions were complete, the tiles on the river edge were channeled to provide a river outlet. So far drainage is going as planned. The river has fallen from 7/7 water to 4/7 water downstream, and 1/7 water at the edge of the map. This has taken 2 months.
A further progress report will follow once riverbed begins to show up.