I began a new project today, something I'm calling the "God Dam". After realizing that my map is prone to severe flooding due to some unlucky biome placement, and the joining of two rivers, I've begun work on something I hope will solve my massive flooding problem.
Really, the major inspiration was due to the unique way my map floods. The south part of the map freezes later than the central and north part of the map, meaning the south-to-north flowing river has nowhere to flow out. This wouldn't be a problem, except the fact that there is a second river in my map that flows west to east, and falls into the major river via a three story drop. What ends up happening, is the river freezes, and the second river continues to fill the canyon with water until it too freezes, then in early spring, that flood thaws, and continues to flood even worse until late spring when the center of the river actually thaws, thus releasing the floodplain to wreak havoc on my fortress.
This dam solves two problems: 1) The west bank will no longer be cut off from the east, and my migrants won't have to starve anymore. 2) My front yard will no longer be a massive floodplain.
I totaled up all the stone it's going to require, it's a 34x7x9 dam, and it's going to take about 3740 stone to make the entire thing including mechanisms. It's going to be a beast of a project.
Luckily, I'm working on tapping a magma vent, and will be making obsidian to create all of this stone easily without strip-mining my map too bad.
I built a 25x25 chamber, with a 25x25 chamber above that I'm using bauxite bridges to drop water onto the lava, creating an even water distribution over the lava. I think it ought to work pretty well.
As for my question: Has anyone ever dammed a river this big? It's 34 tiles wide.
I've heard of streams and minor rivers being dammed, but not major rivers. Major rivers are kind of a pain because of the likelihood of aquifers. --And I had to get past two of the things (lucky for that magma vent!)