Mount Saint Helens

When we read the geology books, The books teaches that large canyons need millions of years to form through river's current.  There is a problem. This teaching is not necessarily true.
A very significant event  occurred in the state of Washington. Mount Saint Helens  erupted. At 8:32 in the morning, an earthquake of magnitude 5.1 occurred near Mount Saint Helens. The mountain collapsed into Spirit Lake causing a giant water wave over 800 feet height onto the north shore of the lake. Following the event came hot gas mixed with volcanic fragments. The flow moved down from the mountain at speeds of over 70 miles per hour with temperatures over 700 degrees. There were mudflows surges because the snow and ice on top of the mountains. These mudflows surged off the mountain at 50 miles per hour with a force that extended for over 20 miles past the mountain. What was the effect of all this devastation?  Many large canyons were created in a short time. One of these canyons was nicknamed the little Grand Canyon. because its formations resemble that of the Grand Canyon.
The question is, "How long did it take to make this canyon?" The answer is, one day. This fact should call into question the necessity for long ages to make large canyons.