It's like this: When glaciers melt they leave a lot of water. That water has to go somewhere.
At the end of the last ice age, the water from the ice sheet covering northern Washington, Idaho, and Montana (oh, yes, and Canada) was trapped behind a finger of the glacier, forming a large lake (Glacial Lake Missoula).
This ice dam could not hold back all that water indefinitely, especially as it was melting, too. One day, the dam holding back the lake burst, and the entire lake poured forth into western Washington State. It is possible that the entire lake drained in 48 hours. Can you imagine a 2,000-foot-high wall of rock, ice and mud thunderng towards you at 65 MPH1?
This water flowed in a sheet across the entire western half of the state, down the lower Columbia River, and through the Cascade Range into the Pacific Ocean. The water couldn't empty into the Pacific fast enough; it flooded the Columbia River Valley, and temporarily backed far up the Snake River and Willamette River valleys.
This happened not once, but up to 40 times between 20,000 and 10,000 years ago. Somewhere in the middle of all that, Lake Missoula's southerly cousin Lake Bonneville overflowed the Great Basin and poured more water and sediment into the Snake River and Columbia River valleys.
The flood left 200-ton boulders scattered like toys and 30-story-high piles of gravel. Even more amazing was the gouging of many deep valleys in western Washington, now called coulee
s. These valleys give the Channeled Scablands their name.
In a bizarre echo of Glacial Lake Missoula, the lake behind the Grand Coulee Dam fills the largest coulee in the region.
These features exist nowhere else on Earth (although something like it has been spotted on Mars).
The idea of such huge geological features being carved out in such a short period of time was so inconceivable that the theory of the Channeled Scablands was ridiculed when J. Harlan Bretz proposed it. However, it is now generally accepted.
1About 105 KPH. Think Metric.