A floodgate is an object used to regulate the flow of fluids, such as water and magma. They can be manufactured from rock (at a mason's workshop), wood (at a carpenter's workshop), metal (at a metalsmith's forge), or glass (at a glass furnace). Once manufactured, they are built by a miner in the desired location by using the keys , . They are necessary elements in most irrigation schemes.
A floodgate in its default position (closed) prohibits the passage of fluids and all other creatures. A floodgate can be linked to a lever, a process consuming two mechanisms (one is installed at the floodgate and the other at the lever itself), and then when the lever is pulled, the floodgate will open.
Floodgates do two things:
- If adjacent to the cave river or magma flow, they pull water or magma into adjacent channels;
- If adjacent to a filled channel, they pull liquid out of it and into the world, causing a (hopefully controlled) flood.
- Note: Floodgates can only be placed inside the cave. You cannot put them next to the outside river, for example.
An open floodgate essentially becomes a channel that allows water to flow through it until it is closed again.
WARNING: Floodgates are particularly dangerous in that if a source of water passes through a floodgate, it becomes infinite. If a floodgate is allowing water into a room, and then the water from that room escapes into the cave at large, the water will not stop at any point and the entire cave will flood with water, as will most of the wilderness. The same is true of magma.
Floods originating from a floodgate will be stopped by a channel. The channel will be filled, but it will not run over the channel. Thus channels can be used to protect from being flooded by lever 'accidents.'
 De(con)struction and Safety Measures
If a floodgate is ever removed (by one of your workers) or destroyed (by an enemy), a one-tile channel will be left in its place. As you cannot place floodgates on channel tiles, a destroyed floodgate may not be replaced.
Some creatures and monsters have the ability to destroy buildings. If these monsters can also survive under water or in magma, they may crawl up through a channel and destroy floodgates. If a floodgate is destroyed by force, it will also spawn a flood event. Many a fortress has drowned in magma due to a magma man destroying a crucial floodgate. As monsters move to their next target in as straight a line as possible, you can protect your floodgates with traps, like this:
▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒≈≈≈ _X_^^^^^^____XX≈≈≈ ▒▒_▒▒▒▒▒▒_▒▒▒▒▒≈≈≈ ▒▒________▒▒▒▒▒≈≈≈ ▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒≈≈≈
Monsters may destroy the two outer floodgates, leaving them in a permanently open state. The flood will be caught by the channel. If they ever try to get at the third floodgate, however, they will have to walk over a series of traps. Usually, even a single weapon trap ought to suffice; but better not to take any chances. You may want to add a maintenance access to this setup so that your mechanics can deal with stuck traps.
Some river, chasm, and magma creatures are a threat to be reckoned with, and who knows what strange beings may show up outside your fortress? Therefore, you should consider security measures whenever you build channels.
 Use as a Wall
For most practical purposes, a floodgate is a wall on top of a channel. This means that if, for instance, you have to dig out a bedroom's walls (possibly to get a cluster of sapphire to placate a dwarf in a strange mood), you can plug the holes with floodgates to restore the original room's shape. Floodgates act as walls when making rooms. The only limit to this behavior is that trolls (and some of the other large monsters) can destroy them.
 Doors as Ersatz Floodgates
If you need to channel a fluid over a long distance, you may want to just use a long narrow corridor instead of a channel. In a corridor, you may also want to employ doors instead of floodgates. After all, doors can be destroyed and replaced as often as necessary. However, doors are not a full replacement: they will hold back the flood as long as they're closed, but once they have been opened, closing them again will not generate an antiflood event.
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