First, you went from a prolonged severe drought to a very intense rainfall in a very short amount of time, extremely not helped by the massive surface runoff when the emergency spillway was put in use. I don't know all of the geological processes involved, but the upper soil layers were probably made far more mobile and less stable by the drought.
Second, the massive erosion is happening on a slope, whereas the Grand Canyon is (comparatively) flat. A steeper gradient means less force is needed to set sediment in motion.
If you want impressive floods, the Bonneville and Missoula floods are the largest known floods in history. Imagine floods with hundreds or thousands of times the flow rate of Niagara Falls, which cumulatively scoured the Columbia river region by hundreds of feet.
Yeah, the ice age water flows were quite something: similar to the spillway in question just slightly bigger, a natural spillway of Lake Agassiz, River Warren have first cut the Traverse Gap, a mile wide and 130 feet gap into the Big Stone moraine and then created a riverbed five miles wide (!!!) and 250 feet deep.
First, you went from a prolonged severe drought to a very intense rainfall in a very short amount of time, extremely not helped by the massive surface runoff when the emergency spillway was put in use. I don't know all of the geological processes involved, but the upper soil layers were probably made far more mobile and less stable by the drought.
Second, the massive erosion is happening on a slope, whereas the Grand Canyon is (comparatively) flat. A steeper gradient means less force is needed to set sediment in motion.
If you want impressive floods, the Bonneville and Missoula floods are the largest known floods in history. Imagine floods with hundreds or thousands of times the flow rate of Niagara Falls, which cumulatively scoured the Columbia river region by hundreds of feet.