Lake Missoula
Lake Missoula was an enormous glacial lake in the American northwest, which was formed as the last Ice Age was ending. It periodically discharged into the Pacific Ocean, reshaping the landscape and in the process creating what is now confirmed to have been the largest waterfall in the world(a).
(a) https://www.bbc.com/travel/story/20170601-the-largest-waterfall-that-ever-existed
See: Deglaciation, Lake Agassiz, Melt Water Pulses & Ronnie Gallagher