40d:Water flow

Water flow is an informal term used occasionally by players to denote the ability of specific water tiles to power a waterwheel. The direction of the flow doesn't matter for purposes of powering a wheel - if it's there, it's there.

While there obviously is a game feature at work, the specifics are unclear and unknown at present.

There are two ways to tell if water has "flow". One is to build a waterwheel and see if it works - time consuming at best, whether for your dwarves or you as a player (if you save the game to test it, then reload once you have your answer). The other is to keep SHOW_FLOW_AMOUNTS, in init.txt, set to NO (the default). Then any flowing water will appear as alternating between a blue ~ and ≈, but only when the game is unpaused. (If the value is YES, then you will see the depth of all water, but not flow.)

Natural flow

Rivers and brooks always have flow. Brooks require that you channel down through the surface to place a wheel, but the flow is there. With dwarf-made bodies of water, the existence of flow is less predictable.

Channeling moving water

Creating water flow seems to be (semi-)reliably created where a channel is dug from a brook or river, back to the same brook or river with no diagonal connections. For example #1 will create flow, while #2 will not and #3 may or may not.

    ≈  River or Brook
    ~  Channel 
#1     
     ≈~~~    
     ≈  ~  Flow
     ≈  ~
     ≈~~~

#2
     ≈~~~
     ≈   ~ No flow (due to diagonals)
     ≈   ~
     ≈~~~

#3
     ≈~~~
     ≈  ~~~ Possible flow (not perfectly understood)
     ≈  ~~~
     ≈~~~

Creating flow

If water is moving from one area to another, there is flow, at least until the levels balance out. This can be done with drainage or pumps. So long as the water is moving, there is flow.

Creating permanent artificial flow

Pumps can create flow in a stable body of water as well. If you pump from one end of a body of water and water comes in (roughly) toward the other, flow is created in the tiles in between. If you start a pump manually, direct the output back to the other end of that same body of water, and have a waterwheel in place in the flow to provide power to the wheel once the flow has started, a perpetual motion machine can easily be created, with ample power to spare.

Accidental flow in Aquifers

Rarely, when channeling into a level with an aquifer with no adjacent pre-existing channels or water-ways, a single tile of flowing water will be created, enough to power a water wheel. This flow continues even when surrounding tiles are channeled out. With current understanding, this is not predictable or controllable - if it happens, it happens.

(It is theorized that the water that initially flows in to fill that channeled tile somehow gets "stuck" in the flowing state. This is only a theory.)