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[Lormax]

Lets assume major river size.  Doesn't ever freeze over.  What is perhaps the best method to dam it up so that I can work inside the river basin, then open the river back up? 

[Flying Dice]

Simplest way would be to use DFliquids to drop a line of magma across it. Otherwise, you could try digging an alternate route, putting floodgates attached to a lever across it, opening it to the river, and then taking advantage of the lowered levels in the main river to build walls. After the dam is in place, build another line of floodgates downriver from it and do whatever you need to do. Once you've finished, knock out the dam, open the floodgates and close the floodgates on the alternate route

[Lormax]

You mention lowered levels?  How low does it need to be to be able to build something there?

[BigD145]

0 or 1. I think 2+ is too deep...

[noodle0117]

Damming without freezing water is tricky business.
I kinda wish toady made it so that when built wall sections fall, they turn into natural walls rather than deconstruct into stones.
Would make plumbing and clogging stuff up so much easier.

[Flying Dice]

0 or 1. I think 2+ is too deep...

Yeah, you'd probably need a canal at least as wide as the original river to drain off an appreciable amount, possibly more. I haven't had to deal with this for a while, so I've forgotten how exactly to work it.

[Jacob/Lee]

Couldn't you create a long line of pumps going across the river constantly pumping? You can pretty much completely stop a stream with that, I imagine you could do the same on any size river.

[Flying Dice]

Couldn't you create a long line of pumps going across the river constantly pumping? You can pretty much completely stop a stream with that, I imagine you could do the same on any size river.

From the sound of things, the OP is talking about a major river, which can mean upwards of 50 tiles wide. I don't think that pumps along the banks would be able to completely drain it, and even if you put enough in for it to be possible, you'd still need somewhere for all that water to go, as well as the power to run them all.

[Leonidas]

I would divert the river down into the first cavern. If you plan it properly, you won't lose any miners and you can stop the draining whenever you want. Your FPS would stink, but you would drain the river.

[Kweri]

What I've done in the past has been to figure out which end of the river is the "source" end and which is the draining end. At the source end, go down z-levels until you have a rock layer. Clear out an area as wide as the river and a few tiles long (directly under your river). Turn those edge stone tiles into fortifications - we'll be draining through them. I also build some raising (not retracting!) bridges across my drain edge and hook them up to levers so I can control whether the drain is open or not.

Now we're going to use a cave-in to open up the river to the drain - where the end (furthest from the drain edge) is on the area you cleared out under ground, make sure the last row is dug out on every z-level up to the one directly under the river. Cave-ins will collapse through floors and this will let the water flow down your chasm, into your dug out area, and if the raising bridges are down - out the edge of the map.

Build a wall on the edge of the river, add a ramp or stairs so your dwarves can get on top, and then construct a line of floors from there that is directly over your soon-to-be chasm area and crosses your entire river. Deconstruct something to cause those floor tiles to collapse (make sure nobody is standing on or under them). They should fall and collapse through all the mined out floors of your chasm area.

Now that the chasm is open, the water can flow immediately down and then back off the map. Flipping your bridge levers controls whether your river is "off" or "on" further downstream. It may take quite some time (and possibly eat some fps) when you go from a drained river to a flowing one again, but it'll get there.

I've used this same idea to drain oceans so I could work on the ocean floor on an embark that only had one embark square of land, the rest being water. It took ages but it felt awesome once I finally had control over the ocean itself! 

[Leonidas]

What I've done in the past has been to figure out which end of the river is the "source" end and which is the draining end.

One way to do that is to dump something with bloodstains into the river. The blood will wash off and then flow in the direction of the river.

I drain with a different method from what Kweri suggests. I guess cave-ins always scare me, and I usually try to stay off the surface.

I dig the drain and then have a miner dig a ramp, which punctures the floor above and starts the water flowing. You prevent the miner from drowning by putting a floor grate on top of the drain.

[Triaxx2]

What's the Z-level depth of the river? The deeper the river, the trickier it can be to drain it. (And the more FPS consuming.)

My method is to simply cut a sharp channel two wide, two spaces away from the river. Drop in 8 pumps on either side, and power them with a waterwheel and two gear assemblies. Run the channels down past your working point, and pumps at the end to pull the water back into the river. (Trying to directly connect to the river doesn't work, unless there's a waterfall in place.) The idea isn't to drain the river, not yet, but just to induce a pair of shallow spots. Build walls, and then move the pumps up. Repeat until you've got enough space to move the pumps in and repeat the process. It's slow and time consuming, but it will get the job done.

Alternately, go down to magma, piston some up, then pump it up into an over head bar with a hatch bottom and pull a lever to drop the entire line and create the first row of an instant dam. (You might want to cut your pass around first to avoid major flooding.)

[Particleman]

I've done this several times. What I do is build a line of pumps across it (not along the bank, OVER the river, like a bridge) powered by waterwheels and pump all the water back upriver. Because of the way DF handles this kind of thing the pumped water basically disappears, which causes the riverbed behind it to dry up so my dwarves can get down there and build walls and/or floodgates and such.

[Leonidas]

This sort of question comes up so often, maybe the wiki should have a page about it.

[BigD145]

What's the Z-level depth of the river? The deeper the river, the trickier it can be to drain it. (And the more FPS consuming.)

Rivers have depth >1? I've only ever seen 1.