Twenty inches of rain in 24 hours is extraordinary, even in Washington state. I lived there through some major storms in the past, and the flooding isn’t caused by logging. TWith enough rain, the landslides happen anyway, but are more visible in clearcut areas. The rugged terrain testifies to frequent gullywashers and creations, and trees just grow back over it.
You must also consider that unharvested forests will burn in the hot summers and the land will be vulnerable to visible mudslides the next winter.
It is always changing — erosion is natural.
Actually fire hardens the soil, increasing the speed of runoff downstream. The flooding downstream is worse after a fire, but there's less soil in the mix of floodwaters than when clearcut.
More from Colorado's experience with fire: http://csfs.colostate.edu/post_fire.htm