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[CAustin]

First of all, hello everyone.  I've been playing DF for a while now, but this is my first post in the forums. 

Anyway, my current fortress is a good 2 Z levels below the local river.  Obviously, making a tunnel that runs water from the bottom of the river, down 3 Z levels, to a hole under my fortress's meeting area would create a spigot of water that would quickly submerge the entire area.  What I'm wondering is whether or not putting a well over such a hole while it's still dry, then letting the water flow under it, would stop this from happening.  It seems like somewhat of a silly question, given the real-life nature of wells, but stranger things have occurred in the world of Dwarf Fortress.

If this wouldn't work, then what would be the best way of obtaining a working well in this situation?

[NecroRebel]

Wells do not block the flow of water, so you would indeed be flooded out.

There are several ways of equalizing pressure. The easiest, though somewhat exploit-y, is to have a verticle shaft that goes from the river down to the level you want the well to draw from, then a horizontal shaft with a diagonal kink somewhere along the line, like this:

W+WW
W+WW
WW+W
WW+W

Pressure only transfers through orthagonal passages, so a diagonal like that neutralizes it.

Or, you can have the water fall down to the level below the level you want the well to draw from, then have a pump (whose impassible square totally blocks off the shaft) pump water up to the level you want it on. Pumps set pressure to their level, so, again, you'll essentially neutralize pressure in this fashion.

Finally, and the most difficult method IMO, is to make a reservoir that is totally sealed except the input tunnel, fill it, seal the input tunnel, and then open the well shaft. Once the input tunnel is sealed and no path for the pressure to travel along present, there will no longer be pressure in the reservoir so it won't overflow. You should take care to make it possible to reseal the well shaft, however, as it may eventually empty too far to be tapped, requiring a refill.

[KenboCalrissian]

Yes, you're headed for disaster!  Water will push up through wells, as I've learned the hard way.  (Find the POI called "Blown Wells").

Check the link in my signature for an explanation of fluid dynamics from Kanddak.  Water pressure can be demonstrated with what's called a U-bend, which it sounds like you're pretty close to demonstrating.  Basically, if water goes down several levels, it will come up through any way it can almost to the same level it started.

I was going to say the same thing about diagonals, but NecroRebel beat me to it by a few seconds

[NecroRebel]

I was going to say the same thing about diagonals, but NecroRebel beat me to it by a few seconds

I only know of such things due to the sage works of those who have come before me 

Still, it is very good advice and essential for anyone who wishes to do anything with fluids should know.

[CAustin]

Thanks for the responses, guys.  Good thing I asked before I tried it.  I had no idea about the diagonal trick.  That seems useful for when I'm in a pinch, but I think I'll go with the pump method instead just because I can.  I was already thinking about doing that anyway (although I wasn't quite sure if even that would work) in the case that the well did in fact allow water to flow through it.  Thanks again, everyone.

[Skorpion]

Short answer, yes.

Long answer, yes, BUT.
Water rises to one level below the source, generally.
HOWEVER. If your well is on the same level as the river's water and you put extra water into the river, YOUR WELL WILL FLOOD.
One of my forts had that problem, so it's very confirmed.

Now, my solution is to make the well where it's above the level of the river.

[Firnagzen]

I think I'd best elaborate.

Water paths preferentially in this order: Down, horizontally and lastly vertically. As such, a river, which has high drainage at one end, will fill a U-bend to just below its own water level, because the water would rather path to the nearly infinite drain (horizontally) than to its own level. However, block the river, or add more water than the drainage can handle and that last z-level will be filled. Read Kanddak's explanation of fluid dynamics, as Kenbo suggested.

To KenboCalrissian: Yay! Someone joins me in my crusade against ignorance of hydrodynamics!

[CAustin]

Firnagzen, I think I actually used that dynamic in previous fortresses that had wells on the same level as the water source.  Unfortunately, the area I'm currently building in is almost completely flat, so I basically had to dig a pit as my fortress's entrance that went a couple levels down (there were two floors of nothing but clay on top).  I actually read a whole lot on how fluids work in the game (mainly from the wiki), but for some reason I couldn't find anything on the well issue.  Maybe I should add this to the wiki just for good measure?

[Firnagzen]

It's simply a rather specific case of the general rules, of which Kanddak's writeup is the most detailed version of. And frankly, the pressure page on the wiki needs an overhaul, but I can't bring myself to do it...

[Thief^]

'course, with the kind of pumps we have in DF, in reality water pressure would push water through them quite easily.
... and with diagonal passages being wide enough for a dwarf to pass through, they shouldn't cause any reduction in water pressure either.

What would work in real life? Apart from an automatic floodgate system, that is.

[Firnagzen]

Sensor controlled pumps?