Why does the greatest erosion and transportation of sediment by a stream occur during a flood?

When a flood occurs, the stream is getting much more water, and on the parts where the angle of fall is greater and the stream is faster, because of the greater amount of water, and thus force, it managed to take with it much bigger amount of sediments, and also much bigger pieces of it.

The transportation depends on the speed and force of the water, and when there is a flood. both of those are bigger, so the stream is able to carry more sediments over a longer distance, but also over a wider area as the waters are getting out of the normal river flow and flood the valley.