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[FearfulJesuit]

So, if I recall correctly, water in a river is pressurised, unless it's dug into diagonally. I would like to make a dining room with a waterfall. The idea is to build it one Z-level under the river, then dig some channels into it, and connect those channels to the river. They'll drain one Z-level underneath the dining room, then move back up into the river on its other side. Am I courting fun?

[Sphalerite]

How is the water moving back up into the river?  Water doesn't flow upwards on its own, even if pressurized.  You're better off draining the the water into the caverns or off the map edge.

[FearfulJesuit]

But I thought pressurised water could move down n z-levels and then back up n z-levels.

[Sphalerite]

Pressurized water will only move back up to the original level it was pressurized at, and then only if it has no other exit.  In your case the water flowing from the river would only flow back into the river after first completely filling your dining room and all exits from it.  Also, it wouldn't be flowing at that point - it would just fill the available space and then become static.  Also, I'm pretty sure river water isn't actually pressurized.

[Starver]

I've a feeling that any useful waterfall will depressurise any water you let into the top, regardless of how viable the initial pressure is to getting water back to the level you want.  (Ninjaed on this point by Sphalerite)

Any reason you want it to flow back into the river?  Could you send it into a bit of useless (or 'drain-off-the-edge'able) bit of cavern?  Edge-connected cavern sea?  Out through edge-fortifications?  Into magma to create obsidian?  Just into magma, the resulting obsidian falling to destruction on the SMR or just being ignored apart from rubble and FPS-eater?  There's all kinds of ways to rid yourself of the waterfall outflow, apart from just digging a WIIIIIIIIIDE cistern for it to drain into and evaporate at the edges so that it never fills up.

If you're wanting the water back up at the top (in the river, or anywhere else), I'd look at pumping it back up.  Windmill(s)-powered pump-stack.  (Note recent threads about them collapsing... I'd only ever build pump-stacks strictly from the bottom up, even though you can designated to build a second pump over a pre-existing pump-yet-to-be-built.)  Other ways are possible (pump-steps, each of them being worked by the power of a waterwheel on the inward side, similar to the Perpetual Motion machines except without the looping water) if your wind or easy accumulation of multi-windmill power isn't sufficient.

But it all depends on what you want.

The FUN (or ‼FUN‼, or perhaps more likely ≈FUN≈) you will encounter will probably be because you didn't anticipate the water building up where it ends up doing so, and spilling into the rest of your complex.  Build doors, floodgates, and in case they fail, prepare to set down other emergency walls slightly further away (with enough time to get them started before it becomes Dangerous Ground) to permanently seal the area off, and work out which dwarfs are going to be potentially expendible.  If it's your first time trying to breach a watersource, there's plenty of reason to suppose you'll miss something.  This also applies if it's not, of course.

[Saint]

Not pressurized, flowing.
If it had pressure it would explode updward from the river.

[NightlinerSGS]

Also, check the wiki on Pressure http://df.magmawiki.com/index.php/Pressure.
It really helps save dwarves sometimes (but it's also preventing !!fun!!, so use with caution).

[Gizogin]

Another page on the wiki you might want to look at is the one about floods.

[Ross Vernal]

Well, the last time I built a waterfall, I rigged it up so that the waterfall fell through the entrance to my dining room, the bedrooms below it, and so on, all the way down to the lowest ground Z-level, where it was re-directed into another pump stack to drop the river water right next to the input pump stack.

This was then hooked up to a well, and with proper floodgate positioning, I was able to re-route the waterfall into a drowning trap.

[Qwernt]

I tend to use a pump to get the water out of the river, that way I control the speed and not the river.  What I haven't done (but think would be cool) would be to have an "automated" waterfall that only turned on when someone was there to see it. 

[Ross Vernal]

The only thing that sounds hard about that is finding where the best pressure plate spot is to trigger the waterfall.

[Starver]

The only thing that sounds hard about that is finding where the best pressure plate spot is to trigger the waterfall.

In the doorway(s) would trigger on people entering.  And leaving.  And not when standing around.

I suppose if you had one of the one-way passages (one of them for in, one of them for out) you could trigger and untrigger using some external fluid logic.  (A more complex version might be able to tell that three dwarfs went in and only stop when the third dwarf came out, unless a fourth one went in, but needs a lot of effort to support a potentially up-to 120-dwarf dining-room experience. )

Setting pressure plates on every floor-square that isn't chair/table/stockpile wouldn't look too brilliant.  Or maybe it would.  But all of those set up to run a pump to drench a quick-ish draining passage with a water-detecting pressure-plate in it linked to the waterfall mechanism would mean that whilever dwarfs were moving around the (otherwise) empty floor they'd get a show, but risks them sitting down for long enough that they idle-out the mechanism and get no more show.  But I think I'd be tempted to try that method, anyway.