According to geologists, Lake Bonneville was formed from an ice dam during the last major Ice Age on this planet. As the glaciers receded, the dam probably melted and gave way about 15,500 years ago, resulting in a huge torrent of water that craved out the Snake River Canyon and much of the canyons of the Columbia River basin.
It's been speculated that many of the human legends of a Great Flood may be based on humans witnessing the collapse of ice dams at the end of the last great Ice Age causing massive land flooding; the stories got passed down through oral tradition. Oceanographer Robert Ballard (of Titanic rediscovery fame) said in National Geographic magazine that the Black Sea around 9,000 BC rose quite rapidly because of the combination of water coming from the north from glacial melting and the sudden rush of water from the south through the Bosphorus Straits.
The ice dam formed and burst some 40 times.
I don't buy this Grand Canyon formation theory though. There should be some heavy evidence of a flood of such magnitude many miles below the dam location such as plunge pools and ripples.
Also assuming that the elevation of the river bed above and below the dam would be about the same you would think that there would be a waterfall where the escaping water first gouged the river bed below the breech.