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" The hillside near Oso, Wash., that gave way on March 22, killing at least eight (update: now 14, as of Tuesday morning) and leaving another 100-plus people missing, has a history of sliding\u2014both ancient and recent. On Saturday, a torrent of loose organic material, rock, and trees came down a steep embankment right through the Stillaguamish River and into homes on the other side in the small town 60 miles northeast of Seattle. The slide dropped the river level from 3.1 feet to 0.9 feet in one hour, according to National Weather Service data, and roughly 20 feet worth of debris blocked the river. This isn&amp;#039;t the first time this hill has slid. David Montgomery, University of Washington geomorphologist, tells PopMech that the Oso slide was a reactivation of a slide that went several times in the recent and ancient past. &amp;quot;When a slope fails, you create a plane of weakness in the soil and it keeps failing,&amp;quot; he says. &amp;quot;It was a really wet and not very strong slope.&amp;quot; While this mix of fine grain glacial sediment of loose sand and silt is common in the hills of the Puget Sound region, &amp;quot;most hillsides don&amp;#039;t slide down on most days,&amp;quot; Montgomery says. &amp;quot;But when it does go, it goes in an instant.&amp;quot; The most recent slide at this location occurred in 2006. But that followed a history of logging that had left the area void of large-rooted trees. &amp;quot;The slope has a complex history of vegetation,&amp;quot; he says. &amp;quot;If you take the big trees off they do not suck as much water off the ground.&amp;quot;  An aerial view of the hill that originated the mudslide March 23, 2014 between Darrington and Arlington, Washington. Credit: Getty Images When you toss in 200 percent more rainfall in 2014 than an average year, including a soaking March that equates to roughly an additional 15 inches of rain, there was simply too much water mixing with &amp;quot;very weak glacial sediments&amp;quot; to allow the hill to hold, he says. The Stillaguamish River cutting into the toe of the hillside could have played a role too, Montgomery says, although it didn&amp;#039;t really have to with such an unstable mix precariously perched on the steep bank. After the slide, the mix of sand and loose material pushed through the river\u2014it dammed the river with 20 feet worth of debris\u2014to create a quicksand-type environment full of moisture on the opposite bank, which also made rescue efforts nearly impossible in the immediate aftermath. Late winter and early spring represent a dangerous time for mudslides, as buildup of precipitation continues to weigh on the environment. Add in a steep slope without a solid footing\u2014one filled with sand\u2014and the weight of the rain can create a disastrous ending. Tim Newcomb covers sports and infrastructure for Popular Mechanics. Follow him on Twitter at @tdnewcomb. "^^<http://www.w3.org/2001/XMLSchema#string>
<http://www.popularmechanics.com/science/environment/a10202/heres-why-the-deadly-washington-state-mudslide-happened-16629160/>