40d:Flood

There can be up to seven units of water in a tile: 1/7 is ankle-deep, while at 7/7, water fills up to the ceiling. At depths below 4/7, dwarves can walk freely, but if the depth is greater than 4/7, dwarves must swim. At 4/7, a dwarf chooses whether they walk or swim. A dwarf who can't swim can drown in a tile with too much water in it.

A tile will flood if there is a water source adjacent to it. You will get a "Damp stone" warning if the direction you are digging will cause a flood; if you want to ignore the warning and continue digging, then re-designate the tile.

After a flood, the water level in an area will even out, given time. Therefore it is possible to drain a small pool completely without getting too many things wet. Larger lakes can be drained with the judicious use of a screw pump or well-designed networks of channels. Note, however, that rivers and oceans are considered to have offscreen water sources; in order to drain these, you must remove water faster than the flow supply.

Deliberately flooding squares can be useful for farming, particularly on squares that need to be muddied before being worked (e.g., fertile land on the surface, and rocky tiles anywhere). However, water spreads very quickly and floods can get out of control if you don't know what you're doing; if your map has any farmland that does not need to be muddied (such as peat or loam), then it's probably safest to farm there, without messing around with floods.

Water movement is a major source of lag in the game.