Flood

From Dwarf Fortress Wiki

Flooding generally refers to water getting loose and spreading out over an area, usually in an unregulated fashion. While this can be useful in creating plots of muddy ground for farming, deliberately setting any amount of water loose in an area is risky at best, and at worst, will result in lots of fun.

As in real life, water will seek to spread out given open space. In Dwarf Fortress, this means that water will spread until all tiles achieve a depth of 1/7, or it fills the space it is enclosed in. Water at a depth of 1/7 will eventually evaporate, leaving behind a muddy floor. Because of this, it would be nearly impossible to ruin a fortress with water from finite sources such as a murky pool. Note, however, that rivers, brooks, and oceans are considered to have offscreen water sources; their supply has been observed to be infinite, as has that of aquifers. Tapping into one of these without some means of regulating the flow is almost certain to result in fun.

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