Tarqeq: I believe you've found a lake air biome. Merely believe, as oddly, I haven't seen this sort of generation happen myself, but you can check with showbiomes dfhack script.
Interesting! Never heard of it, before! Thanks for the tip!
I had this embark next to a lake, just now, and for no reason whatsoever a huge mass of water got generated on the land and swept my dwarves into the lake to drown, immediately. Could it be that the tree trunks were considered an aquifer or something?
EDIT: So it was just really big mass of water - as if lake water was generated in the land. Very strange. My third group got swimming skill for starters just in case and seems to be holding, so far.
Your third group? Did it happen for your 1st group, or only for the 2nd and 3rd groups?
All seven drowned... twice, because they always died before the flood ran out of steam, so to speak. Turns out spawning on a peninsula surrounded by lake and large mass of water coming from the land can be detrimental to your health.
Third group were just dwarves with swimming, so they managed to survive getting swept into the lake few times.