"To the geologic eye all the surface of the earth is a fluid form, and man moves upon it as insecurely as Peter walking on the waves to Christ." Will and Ariel Durant. Lessons of History (Simon & Schuster 1968).
There is much now that has been posted in other blogs by those who are more expert than I. My initial observations founded in some limited online research were correct. I had observed the scarp of a former landslide as marked by an arcuate band of whitish bare soil. This actually was the location of a previous albeit somewhat smaller landslide in 2006 that ended up sliding into the river course and moving the river course southward with a little human engineered assistance.
For some additional insight on the Oso landslide, I refer you to these blog posts from Dan McShane's Reading the Washington Landscape:
Arm Waving Notes on the Stilliguamish Blocking Landslide
On the geology:
Geology of Stilliguamish Blocking Slide
Excellent historical imagery discussed:
Aerial History and LiDAR of the Stilliguamish Blocking Landslide
Cliff Mass' Weather Blog provides some insights into our wet March that was a factor in this landslide event.
The Meteorological Background for the Stillaguamish Landslide
The tragedy of all this is that conditions conducive to repeated occurrences of landslides were well known and documented.