Lahar from the 1982 eruption of Mount St. Helens.
Source: http://vulcan.wr.usgs.gov/Volcanoes/MSH/SlideSet/ljt_slideset.html (direct link). http://en.wiktionary.org/wiki/File:MSH82_lahar_from_march_82_eruption_03-21-82.jpg
Author: Tom Casadevall