Aqueducts are structures made to carry water or magma across obstacles such as the chasm. They can be made of raw stone, wood, metal bars, or blocks of stone or glass, but only stone, iron, and steel (and likely also pig iron and platinum) aqueducts can carry magma. They must be at least 3 tiles wide, forming the outer walls and an inner channel and can have openings in any of the cardinal directions to allow water to enter or exit from a channel or floodgate.
An aqueduct is built by using/ / / to adjust its size and / / / to place or remove openings in the aqueduct. An aqueduct cannot be placed unless there is at least one opening.
Aqueducts are bugged when carrying liquids across the river, causing unpredictable flooding. To minimize the bugs, keep the "sweet spots" (red) of the aqueduct in dry land:
╬╬╬╬╬╬╬╬╬ Entrance ≡≡≡≡≡≡≡≡≡ Exit ╬╬╬╬╬╬╬╬╬
A filled channel meeting an opening in an aqueduct will fill the aqueduct, and the exit(s) of the aqueduct will then provide a source of flood fluid. Depending on one's intentions, it may be best to provide channels at all openings of an aqueduct to restrain the fluid.
Aqueducts also function as bridges. The walls of the aqueduct act as guard rails – for the water, that is. Dwarves and others can dance on the sides of aqueducts at will.
Bridges can conduct flood fluids safely across obstacles, and are a good substitute for aqueducts in many situations.
An aqueduct requires one stone (or other such material) per four squares. Metal aqueducts require one bar per two squares. Note that perfect efficiency is impossible due to the fact that aqueducts only increase along height and width by odd numbers (e.g. 1x1, 1x3, 3x3, etc.) The maximum length or width of an aqueduct is 9 tiles.
Oddly enough, aqueducts cannot be used to cross water-filled channels. This is probably for the same reason you can't pass floodwater over a bridge over a channel – the game doesn't handle such superpositions.