Landslide as large as 16 city blocks

OSO, Wash.- The aerial photos tell the whole story.

A mile-long section of hillside gone. Destruction so intense it caught the experts off guard.

"I pulled up to it and it's as far as I could see from my vantage point. I was in awe. I've never seen anything like it in my 20 year career,"said Snohomish County fire chief Travis Hots.

That landslide was massive - we are talking one mile of mud and debris. That's the same distance as 16 downtown Portland city blocks.

The pile of debris was 180 feet deep - that's as tall as the Federal Building in downtown Portland, which is 18 stories high.

But now the mounting concern is where all the debris landed. It's in the middle of the Stillaguamish River, creating a make-shift dam.

Eventually all that water has to go somewhere.

"The water is backing up 10 to 12 inches every half hour. That water is backing up to the point it is going to break loose," said John Pennington with the Department of Emergency Management.

That could be a massive flood headed right for Arlington. Some experts worry it could be just as destructive as the flooding we saw after Mount St. Helens erupted in 1980.