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[onodera]

I've strarted a fortress at the condluence of a major and a minor rivers and decided to build myself a lava moat. When the winter came and the water froze, I dammed the minor river with bauxite floodgates where it entered the screen and where it joined the major river. Then I dug out all the ice it it and connected it to a magma pipe with a channel.
When the spring finally arrived, I noticed two things:
1) Floodgates didn't block the water, but only slowed it down. I checked the wiki, and it seems it is a feature.
2) The spillage contacted my magma and formed an obsidian wall. "Fine," thought I, "not that I wanted to open that dam ever again." But then something strange happened: the magma kept steaming behind that obisidian wall and was actually evaporating. By the fall my magme has retreated almost into that channel as was only 2 units deep.

Next winter I dug out the ice again and built walls above both floodgates. Also, as soon as the winter came, the steaming stopped. I also built several more walls between my entry dam and the obsidian wall.

Next winter leaking and steaming continued. Could anyone lend me a hand and tell me what's going on?
http://mkv25.net/dfma/map-3430-crystalwonder

[tp12]

would that be a magma pipe or magma pool?
also, allowing us to see a bit lower would be nice.

[onodera]

That would be a magma pipe.
The level below is mostly unxplored silty clay with my stockpiles far from the magma/water border.

[WCG]

I'm a newbie and have yet to find the magma pipe that's supposed to be at my fortress. But if you're damming a river, without providing any kind of spillway, what else is it going to do? Eventually, the water will rise enough to spill over any walls or dam, right?

If this is the problem (maybe I don't understand your situation), why not build a spillway? Just give the water an alternate route. Or let it pool behind the dam, if you want, but when it rises high enough let it spill over into a channel you've dug. Accept the 'leakage' as being natural for a river, and just design things so it goes elsewhere, not into your magma.

Or let just a limited amount of water into that part of your magma, since obsidian might be nice to have! Just keep it under control, and then mine out the obsidian occasionally.

Bill

[Beacon]

This.

[Osmosis Jones]

Afaik you haven't done anything obviously wrong. This means one of two things; you made a booboo on the idiot list (below), or you have just discovered an interesting new bug, congratulations!

1) Are all the floodgates actually built?
2) Are any open? (If you can see them, the answer is no)
3) Is there an aquifer there? (Very unlikely, but who knows?)

[Fossaman]

Both magma and water can evaporate when they're only one deep. Are you getting enough of a flow to the end of the channel to replace evaporation?

[onodera]

Both magma and water can evaporate when they're only one deep. Are you getting enough of a flow to the end of the channel to replace evaporation?

I don't think it's evaporation. I think the water does seep through the walls or spill over them, but doesn't produce obsidian. I'll dam the main river to find out.

[Porpoisepower]

I can explain the steam...

Non smoothed walls leak.  so water is seaping through to the magma, but probably not fast enough to form more obsidian.

[eerr]

digging such a long trench in a magma pipe drains the top layer
while moving, it can evaporate in any depth

[Dareon Clearwater]

Here's a likely issue: Rain.

I set up almost the exact same situation, I just dammed the rivers with obsidian because the map didn't freeze.  In the process, I learned two things:

1) Magma really moves much faster than it is given credit for (My poor miners ).
2) The bottoms of waterfalls are also river source tiles.

So, I just removed a small portion of the magma channel I made to dam the upper river and let the lava spill out onto the lower.  This led to my next observations:

1) Rain fills river tiles.
2) Falling constructions can really knock dwarves a long way (My poor hunter/soldier ).

The falling rain was producing steam and removing the remnants of lava from the riverbed, but only produced an actual column of obsidian in one instance.