The graphic above shows the scope of the mudslide that swept through a rural neighborhood just east of Oso, Wash., on Saturday. The slide has killed at least 16 people, with more feared dead.
The hill that collapsed, sending a square mile of mud, slurry and wreckage on the river and homes below, had also collapsed in 2006. That previous, smaller slide was nonetheless large enough to block the Stillaguamish River, and large enough to raise some concerns about the stability of the hill, but apparently did not stop homeowners from living in the valley.
Experts have primarily blamed the slide on the near-record amounts of rain that the area has received in March -- more than 7 inches worth, or almost double the usual amount for the month. Much of the area's soil is leftover glacial silt, a loose material that has contributed to other mudslides in western Washington when conditions get wet. One geologist said that such slides were part of the natural process of erosion that has contributed to the creation of Washington's river valleys.
The Saturday mudslide was rather large as far as slides go, however, experts said. And when a segment of land cut loose from the hillside near Oso, it not only buried the Stillaguamish River, but the 49 homes alongside it.