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[zazq]

So i've got a big ol' river, and i want to build a tower coming up though it.  However, there is a river in the way.  How do i block the river, or simply build islands that i can then mine up though?  I heard somewhere that caveins do it, but i've had no such luck with that approach. 

[NecroRebel]

Cave-ins don't work well for damming above-ground rivers because you need natural walls to block things with cave-ins. Since there's usually no natural walls above above-ground rivers, dropping said nonexistent natural walls into the river to dam it is tricky at best.

What you want is a pump wall. Basically, you make a solid wall of pumps pointed upstream across the whole river that pump into the river itself. Power the pumps and they'll basically stop the river short where it goes beneath the pump line. Once it dries, you can then build whatever you want in the river bed; I'd recommend building a drawbridge across the river bed and linking it to a lever so that later you can pull the lever to dam the river on demand.

If your river is big enough and your desired islands/pylons small enough (not likely, but possible), you can also do something similar, except instead of a single-direction wall just make a solid ring of pumps around where you want dry. Again, they'll pump out faster than the water can flow in, allowing you to construct in the hole in the water. A similar tactic is used by some to make holes in magma to gather additional spoiler-metal or to get to the bottom of the magma sea safely.

[rtg593]

Hmmm... So totally doing that on my 26 tile-wide major river taking up most of my embark:D

[khearn]

BTW, in case it wasn't clear, build floors out over the river on which to build the pumps.

[Girlinhat]

Dig a drainage tunnel.  Do this by digging to the map's edge in stone (not soil).  Smooth the edge rock, and then carve fortifications.  Extend the drainage tunnel under the river, crossing Z-levels however you feel is most convenient.  Dig directly under the river, where you get "damp stone" messages.  Only dig, don't channel.  Install floodgates.

Now, above-ground, build a stairs, and a section of floor that extends over the river.  Attach a support, remove the stairs, and pull the lever.

The falling constructed floor tiles will crash through the riverbed and into your drainage tunnel, instantly creating an offshoot area where the river will divert into the tunnel and down the rock, through the fortifications and drain off the map's edge.  Because of this, the river will run dry on the other side, and you can do as you please.  When you want the river back to normal, just close those floodgates you installed earlier.

I've done this a few times.  It turns out to be extremely effective and safe, allowing for instant water diversions and very nice waterwheel rows set underground in the drainage tunnel.  I mostly use it to set cage traps and secure large fish for future aquarium purposes.

[rtg593]

... I have NEVER seen the 'drop floor through river' part before. Pretty sure damming a river is in the wiki, but I'm also sure that part isn't.

It says something about building a bridge under the river, then ramping up from under it.... iirc.

That's GENIUS.

[Girlinhat]

Yeah, stumbled upon that idea a while back.  Basically started thinking "How do I get this river... underground?" and wound up dropping a building on it - and it worked.  I should try to illustrate it or something...

[rtg593]

Yeah, stumbled upon that idea a while back.  Basically started thinking "How do I get this river... underground?" and wound up dropping a building on it - and it worked.  I should try to illustrate it or something...

Put it in the wiki, and in you Dwarven Knowledge thread thingy;)

[zazq]

ha my pump wall is murdering my frame rate. down to about 19fps.  however it seems to be working.

[zazq]

nope, it crawled to 3fps and the game crashed. 

[Nostril actor]

Couldn't you also just build a support a z level over the river and have everything attached to said support, then all you would have to do is to collapse the support. (isn't that what the supports are for in the first place?)

[Girlinhat]

Do tell me, how much experience have you had generating cave-ins?  Your simplistic "why not just" has be wondering.

[NecroRebel]

Couldn't you also just build a support a z level over the river and have everything attached to said support, then all you would have to do is to collapse the support. (isn't that what the supports are for in the first place?)

The reason why you can't do that was actually given, albeit somewhat obliquely, earlier in the thread:

Cave-ins don't work well for damming above-ground rivers because you need natural walls to block things with cave-ins. Since there's usually no natural walls above above-ground rivers, dropping said nonexistent natural walls into the river to dam it is tricky at best.

Constructed walls and floors and all buildings simply deconstruct if they're caved in. Natural terrain doesn't, but as mentioned in the quote there's usually not natural terrain above rivers. The engineering required to get natural terrain where it can be caved into a river would be better served just damming the river directly, so collapsing things with supports is basically just not usable for this purpose.

[DangerDwarf]

Of course if the river were to carve through a mountain then this might be possible. But yeah excellent idea to build the floor above the river Ill def try this with my next fort.

[parlor_tricks]

The cave in method to making holes is pretty known - it was mentioned on the forums a while back too for various odd purposes - Most common discovery of this was when someone ended up collapsing a roof by mistake, and it fell through every floor tile all the way to the bottom, creating a sun roof and usually a lot of dead dwarves.