The mainstream mechanism, developed by Wallace Broecker: meltwater from Lake Agassiz, the ice-dammed lake covering much of Manitoba and beyond, suddenly drained eastward through the St. Lawrence, dumping roughly 9,500 cubic kilometers of fresh water into the North Atlantic, diluting surface salinity, stalling the thermohaline circulation, and cutting oceanic heat transport north. Documented support includes meltwater pulses in sea-level records and modeled climate response. Documented problems: the drainage route and timing are disputed, and a second meltwater pulse at the YD's end did not trigger a comparable freeze.
The folk synthesis: the impact camp notes the timing problem, no documented Agassiz outburst lines up cleanly with 12,900 BP, and proposes the comet itself shattered the ice sheet and released the water, fire causing the flood. The gradualist camp answers that no crater exists and ice sheets collapse on their own schedule. Each side's strongest argument is the other side's weakest evidence.
The claim: Agassiz is the physical substrate of the flood myths, a wall of water across a continent, scablands scoured in days, and per the Carlson reading, the event oral tradition was built to carry. Bretz's Missoula floods, ridiculed for decades before vindication, are the standing parable: catastrophism is sometimes just uniformitarianism that hasn't checked the field yet.
Ice dam or comet: did the Younger Dryas start with water, fire, or both?