Bragging rights for one thing are really no contest. The Bonneville Flood, which roared through what is now southern Idaho about 12,000 years ago was a monster. When ancient Lake Bonneville, which covered most of what is now Utah, broke through a natural plug at Red Rock Pass it sent water crashing down the channel of the Snake River five or six times the flow of the Amazon, tearing out chunks of canyon the size of cars and tumbling the rock into rounded boulders. It drained some 600 cubic miles of water into Columbia and out to the Pacific in a matter of weeks and is said to be the second biggest flood in geologic history.
Second biggest. So, who had the first? Northern Idaho, of course.
About 15,000 years ago, during the last ice age, a huge glacier blocked the flow of the Clark Fork River near where it enters Lake Pend Oreille. Water backed up into present day Montana, forming an expansive lake that geologists call Lake Missoula. The glacial lake covered 3,000 square miles, with a depth of up to 2,000 feet.
The ice dam that created Lake Missoula could not contain it forever. When the ice finally gave
way--perhaps in the period of a day or two--a massive flood resulted.
You could not have outrun the rush of water called the Spokane Flood. It came ripping out of Idaho and into Washington at up to 80 miles per hour with the force of 500 cubic miles of water behind it. The flow may have run at 13 times the output of the Amazon. It's no wonder it scoured out 200-foot-deep canyons, and ripped the top soil away across 15,000 square miles of what is now Washington State.
The Bonneville Flood happened only once, while the Spokane Flood may have happened again and again—maybe up to 25 times—while ice dams formed and broke away.