Aerial and LiDAR images explain a great deal about the landslide that blocked the NF Stilly (geology-of-silliguamish-blocking-slide and arm-waving-notes-on-stilliguamish). The LiDAR particularly shows the extent of past sliding and the scale of the past landslides. This leads to policy issues, but I will save that for a later day or two or week.
This image is from 2003. As can be seen the river and slide area has shifted a bit westward. This image is before the slide that took place in 2006 that blocked the river channel in a very similar manner to the Nooksack River blocking event of 2014.
LiDAR is a great tool for assessing landslides. The bare earth images have made mapping large landslides and large landslide deposits an office exercise with focused field trips. The active slide area is easy to see, but there is so much more to be gleaned from the LiDAR in regards to scale.
This next LiDAR image pulls back away a bit to show the bigger picture of the slope stability of the area.
The larger red outlined area in the central portion of the image is from a much larger slide. The deposit area from that slide was even bigger than what is indicated, but the southern end has been eroded and removed by the river. It would be very interesting to know how old this slide is. And also might be informative to look at the geometry of the headwall area.