First, it's Mount Saint Helens there is no apostrophe.

Sure the channels were filled up with dust. But there was a pyroclastic eruption which created super heated mud flows. The eruption blew off a massive amount of the cone creating a new lopside crater. Mount Saint Helens was a perfect Shield Volcano prior to the May 1980 eruption.

If on Mars gullies and channels are created during the pyroclastic flows it is probably because the surface is cover with fine dust which is pushed out of the way of the blast. Or permafrost is releaved of it's water in a similar manner as the mud flows happened on Saint Helens. But, Mars.has plenty of dust to be pushed and the prorclastic flow could carved new channels.

--Chris Vancil