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40d:Dam
A dam can be built across a river or brook. This has several uses:
- Stop a river from flowing altogether
- Modify a river's path
- Create a pretty lake or functional reservoir
- Create water-based traps
Dam Building
Finding the direction the water flows is probably a necessary step to take. This is actually fairly simple - at one end, the river falls off the map, and so is of a low water height. Check the outmost tiles of both ends of the river.
Ice Method
The easiest and quickest method of Dam manufacture is only possible if your map freezes during winter. Simply dig out a thin section of channels across the river and build a wall out of floodgates, blocking the path of the water.
Draining method
Another way is to drain the river. This can be done by building a tunnel next to the river that dumps into a chasm or deep valley. An alternative method to this is to use a large number of pumps to move more water out of the river than can be replaced by the water flow. For a river with a width of 4 tiles, you will need three on either side pumping it into channels or tunnels, ideally off the map, a waterfall, or somewhere where flooding will not be a concern. While the pumps are operating, it will be drained enough to build the dam.
Magma method
If you have access to a magma source, you can pump magma into the flowing water. When the magma and water collide, the result is a tile of obsidian- an added bonus if you're in the market for making obsidian items. Pump enough magma into the river to reach both banks for a stretch at least 3-4 tiles wide in order to give your dwarves enough space to channel out the obsidian and place floodgates.
Beware: Pumping magma too close to the edge of the map could result in permanently damming your river! Obsidian tiles created on the edge of the map cannot be mined out!
Actual building
Once the river has been drained you may have to dig into the riverbed to be able to place buildings. It will look a bit like this:
Once the dam has been built and when the water comes back it will fill up the gap as far as it must to let the blocked water flow further. If you build your dam up to the level of the river the water will stop at the top level of the dam (it will not overflow). Since this defies logic it may be fixed in future releases.
You can create a massive flood if you build a dam out of floodgates and open them all at once.
Further possibilities
A similar technique can be used to create water "holes" in large bodies of water, such as lakes or even the ocean. Build a ring of floors at the water's surface, and line it with an immense number of pumps, facing out of the area you wish to drain. The pumps should be able to push out water faster than it can come back in, allowing you to build a dike in this area. This is hazardous if the water is more than one z-level deep, since it's highly likely a dwarf can drown. Even if technically safe in that the water level doesn't rise high enough to be dangerous, the rapidly-fluctuating water levels may cause dwarves to repeatedly stop work at the site anyway.