That's not true. You might be thinking of the Missoula floods that carved out the channeled scablands of Eastern Washington. But the Grand Canyon's river is generally the same size it always has been, and slow erosion forces created it.
Why not both?! It's not ridiculous to allow for constant, steady erosion and the occasional 10,000 year flood shenanigans!
Example from in our great-grandparents lifetimes. There's a cool place called the Bridge to Nowhere in Southern California => https://goo.gl/maps/XMerBpT3J2caLJ696 / https://en.wikipedia.org/wiki/Bridge_to_Nowhere_(San_Gabriel.... The bridge was built as part of a massive project to build roads through the San Gabriel Mountains in the mid 1930s. Only a couple years after the bridge was completed, there was a massive rainstorm washed away most of the newly built roads. The bridge stand about 120 feet, roughly 36 meters, above the river below. I was talking to a park ranger who said there were reports that the flooding nearly reached the bridge. While this bridge stands over a river that's been slowly eroding the valley below for (millions of?) years, every now and then Mother Nature says "I'm bored, let's hit the biblical flood button and see what happens!" What kind of boulders could a violent rush of 20m-30m+ flooding move? Big ones I'm sure! Who the heck knows what kind of freakish rain storms or natural damn bursts have happened in the time that the Grand Canyon has been forming!
For anyone in the SoCal area, the Bridge to Nowhere is a fun day hike. It's about 5 miles (8km) one way from the trailhead. It's a very cool hike. If you're going in summer time, bring plenty of water, sunscreen, and some head protection. It gets toasty in that canyon.
Sure, there are certainly big floods now and again. But the point stands that the grand canyon's formation was primarily from slow erosion, not from big floods doing the bulk of the work.
