This time I constructed it as usual and sent a miner to punch through into the underground river. The entire hallway immediately flooded to a depth of 7 with no warning, and drowned the miner (without moving his body, so there was no water movement). It was instantaneous. EDIT: For the sake of clarity, previously, water ran in at a normal rate - this was effectively "water teleporting".
I did extensive pressurized water experiments while trying to develop a carp cannon (back when carp were still hardcore) and that's what I'd expect when you breach a lake from beneath -- water from above lands on water below, creating a x/7 water block sitting atop a 7/7 water block:
Pressure. This allows the water above to teleport through the breach straight to the end of the stream. The draining won't even slow down until a significant area of water around the drain is below 2/7.
I tried to put a wall through an underground lake with pumps and couldn't do it. You can do so with a brook but not an underground lake. An underground lake covers big areas and connects to wide stripes of map edge. Very difficult to put a dent in it.
Something similar happened to me when I forgot how diagonals worked:
As fast as you can say *fworsh*, the room was full... That sure taught me. Either breach the lake diagonally, or channel from above. Or some dwarfy arrangement of levers and pillars to punch a hole from above...