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Major landslide in the state of Washington, USA

Oso, Washington, is at the marker. Arlington is to
the left, Darlington is to the right by the magnifier bar.
The slide at Oso cut off Highway 530 between
Arlington an Darlington.Take I-5 north from Seattle about 50 miles through Everett toward Arlington, and turn east onto Highway 530, which takes you south of Mount Baker. Along this road is the small town of Oso, population about 200. At about 11:00 this morning, a massive slide of mud, rocks and trees travelled a mile down near Oso, taking down at least 6 homes, killing at least three people and trapping others. Three more are reported in critical condition.The slide was at the 29400 block of SR 530 near milepost 37, between the cities of Arlington and Darrington. It landed in the path of the Stillaguamish River, reducing its level at one spot from about 3.1 feet to 0.9 feet, indicating that the slide appreciably blocked the river. The state hydrologist reported that 15-20 feet of debris blocked the river, and that its flow had been reduced to about 1,000 cubic feet per second. Other reports have said that the slide is 135 feet and 180 feet deep.
In situations like this where an earth slide blocks a river, the concern is that water will pond behind the blockage forming a lake, and that the blockage--a dam--will suddenly collapse and release the water catastrophically. The National Weather Service has said that for this to occur the water would have to be blocked for 36 hours, and then released within an hours time.
Snohomish county officials have advised residents downstream of the slide to evacuate their homes as "It is going to break loose and the question is how and where" (John Pennington with the Department of Emergency Management (quoted from King5 news here).
The National Weather Service has issued a flash flood watch through Snohomish County through Sunday afternoon. The ground is saturated with water from recent rains, flash flooding is possible, and the saturated ground combined with rain is believed to be the cause of the Oso mudslide. Flood alerts have been issued both upstream and downstream of the slide.

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