40d:Ice

Ice is water that has been frozen by cold temperatures, usually because of winter. Any water that is outside will freeze during winter, while it will stay the same if it is inside.

A tile of water will freeze into an ice wall, while creating an ice floor one Z-Axis above it. The ice wall will be the same no matter how deep the water is. The floor can be walked on freely, and there doesn't seem to be any kind of thin, breakable ice. The ice wall can be mined through, which leaves the above ice floor intact. Channeling the ice floor will destroy both the floor and the wall. The ice wall will apparently melt into water of depth 7 [1] when warmer temperatures arrive. Presumably the ice floor will melt as well, but whether it turns into water or just disappears is unknown at the moment.

Ice can be mined through without causing a flood, making Winter an ideal time to get rid of any lakes that are in the way.

Ice as a Stone

Ice appears as a light blue stone which can be found by mining through an ice wall. Ice can be used to build constructions and workshops. Ice boulders and objects made of ice will melt when exposed to warmer temperatures (such as inside a fortress), giving it rather limited use. Nevertheless, workshops made of ice have a certain novelty to them, and it's even possible to make furnaces out of ice, as counter-intuitive as that sounds.

The games refers to Ice boulders as "water". It doesn't appear in any stockpile options or the manager, but it does appear under the "stones" section of the Stocks menu.

Out of Water

As ice takes the place of water, entire water supplies can be completely frozen on colder areas. This can depopulate an entire fortress in just the first winter if they're unprepared. The only way to counter this is to make a water pit inside beforehand, or brew enough drinks. Oddly enough, drinks do not require water to make.