at first i thought it was elderberry, but then in the second picture it looks different. i dont think thats it.
i also dont think its tickseed either (coreopsis) with the sticky seeds, but i could be completely wrong cause i am not that familiar with those species.
well sorry thats wasnt very helpful, but just guessing at the name that plant game.
after looking up some pics just for curiosity sake, i think the OP could be right, it could very well be water hemlock.
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