Thawing river with lava underneath causes obsidian to form

[Dreamyr]

What I assume to be happening is that the thawing ice falls breaking the floor and exposing the water to lava. Given my objective here was to thaw the river and not make an obsidian farm this is a problem. I honestly don't think the ice should be breaking the floor otherwise what's the point of doing this in the first place.

[Terminal]

Quick question - is the floor between the river and the lava channel natural, or constructed? I can think of two things that might be happening here.

The first is this - when the ice melts, if the floor is natural, what happens is it turns from a dry tile into a DAMP tile. I have no idea if being exposed to a damp tile is enough to turn lava into obsidian - it seems weird and it would be new to this version of the game if so - but melting ice does not fall across Z-levels. It just turns into a 7/7 tile of water on the spot, so there's no reason for it to place water in the channel below.

On the other hand - if the floor of the river is constructed - what might be happening is it gets DESTROYED when the water freezes, and then when heated it results in there being no floor between the river and the lava channel. I am pretty sure this is not the case - I think even in this scenario the ice would leave behind a patch of furrowed ground rather than destroying it entirely - but it's the only other thing I can think of.

More information would be helpful.

[Dreamyr]

It's a natural river with natural tiles. I dug out a canal under it to thaw it as the wiki says that lava underneath or beside water keeps it from freezing.

xrrrx
fffff
xlllx

x= blocks
r=river
f=floor
l=lava


I constructed it in the winter and the first time it worked fine even though the surface didn't thaw, the actual tile itself was water. The second winter when I already had the lava under the river when it started to freeze was when I started having problems. maybe it's the floor tile on top that fell? I found out about it because I was getting 'cavern collapsed' messages IIRC.

[Terminal]

If a cavern collapsed there is definitely a breach somewhere along the length of the river, though good luck finding it. If the obsidian has not been mined out yet, you should be able to identify the breach spot by individually checking each river tile for a rough obsidian stone floor.

[Dreamyr]

Ah, well I'm kinda demotivated about it right now so we'll see if I go back to it. I quit in frustration over it last night and went and started a new fort that gave me a first wave of immigrants with two necromancers and a legendary stonecrafter.

Then I promptly overwrote the autosave after that immigrant wave just a little bit ago trying to get back to the point I had my lava problem with my first fort. Fort autosaves really should be separate, I'm just a mite salty right now.

[nrusselluk]

one thing I remember about building under rivers.
if you build a block under a river, when you remove it, the floor above disappears too.
a feature i've used numerous times.

[Dreamyr]

Ok, something weeeeird is going on with this other world I started after my lava-river project induced depression. I just got my first wave of immigrants and there's necromancers in it, again.

THREE of them, including the legendary stonecrafter who was a normal dwarf last time.

Yes, I remembered to save this time too.