- Stop a river from flowing altogether
- Modify a river's path
- Create a pretty lake or functional reservoir
- Create water-based traps
A river will not "overflow" its banks above its starting level because of a dam. (Note - if a waterfall is upstream, the upper level is the "starting level"!)
 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.
A time-consuming but very effective and nearly guaranteed way to build a dam is to "part the waters" in the river or brook. This construction is easiest to perform on a brook, but can be done on a river, too. See the diagram.
- For a brook: Firstly, at some point upstream, channel out a 3x2 tile from the brook, and build two water wheels that run along the water. Do not forget to support the waterwheels with an axle first. Then, further downstream, build a row of screw pumps that pump the brook's water in its opposite direction, e.g. towards where you built the waterwheels. Because the brook has its own special "tiles", water will not immediately enter the river again, and can flow freely on its "surface." Next, build walls to contain and direct the water exactly in the opposite direction of the stream, for some 15 tiles. Then, channel the brook here, so that the pumped waters may return to the brook. Next, repeat the procedure in the exact other directions, some tiles away from the first row of screw pumps. Then, draw the power from the waterwheels into your rows of screw pumps, and the procedure is ready to start.
- For a river: Do exactly as stated above, except that you have to build floors over the river to support the screw pumps and the redirection of water flow.
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Once the screw pumps are started, they will pump out water and dry out a little valley of riverbed inbetween the rows, mimicking the narrative of Moses parting the sea, hence the method's name. This valley is where you will build your floodgates. Simply dig out a downward ramp leading to the riverbed, and start designating a row of floodgates. Often, there will be tiles with a depth of 2 of water left, prohibiting your dwarves from designating a floodgate there. Simply unpause and pause the game to let the water fluctuate, and designate it once it is cleared.
Once designated, your dwarves will repeatedly cancel the floodgate construction jobs (Dwarf cancels Construct Building: Submerged) because of water fluctuations arising. Simply pressclose to the floodgate, and remove the job uspension. You will have to do this repeatedly - don't worry, your dwarves are building the floodgate, they are just being interrupted frequently! Eventually, after many interruptions, the floodgate will be constructed. Proceed to repeat the process with the other floodgates. Once they are all built, you will have an effective dam.
 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. The dumping tunnel should only be one square wide, with a single floodgate to control it, as long as where the water first enters it has a fairly large number of squares of "Open Space" to move into. The water over Open Space will rapidly force its way down, forcing the rest to flow at warp speed. Be sure the floodgate is operational, as this sort of flow imposes terrible game lag while active. The more edge tiles drained from, the faster the level will go down enough to allow construction.
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.
If your dwarves are persistent enough you can also dig out a large chamber under the river at Z-1, channel out holes on both sides at Z0, and cover these holes with walled-in bridges. When you raise the bridges the water drops down; when you lower them it starts flowing again. This avoids the 'infinite hole' bug that can result if you use collapses to punch holes in the bottom of brooks.
 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!
If you wish to move magma over a brook without damming it, constructed floors over the brook tiles work.
If your river is underground, you can use channeling to collapse a section of rock into it. If this rock was constructed (i.e., you built it as walls or floors), it will revert to its component stones and sink to the bottom, not blocking the river at all. However, if the rock is natural, it will retain its shape and block the flow of water.
Since above-ground rivers normally do not pass beneath natural rock, this strategy is difficult to apply to them. But dwarven ingenuity should never be underestimated.
 Your Dam
 The Dam
After your dam has been completed and looks something like the image, you can expect water to just stop flowing UNLESS there is a waterfall. If that is the case, the water will just flow around it after flooding a fair amount of space around it. If you messed up while making a magma dam and permanently obstructed the edge of the map, you can expect the entire world to flood up to the point where water enters.
 Actually Building
Once the river has been drained (which can take a VERY long time), you may have to dig into the riverbed by ordering a channel to be dug in the open space tile above the tile you want to build on. The result of this, however, is that the tile is no longer considered a brook/river and it behaves like any other storage of water. It no longer colored a pretty light-blue and nothing (including dwarves, caravans, goblins, etc) can walk on top of it. This can be very annoying, so be prepared.
 Further Possibilities
- Most of the above techniques can be used to puncture oceans and lakes, for dikes or random under-water bunkers.
- You can create a massive flood if you build a dam out of floodgates and open them all at once. Good for traps. Works with magma.
- Being in control of your local river/brook makes you more of a