As part of the natural system in the Pacific Northwest, rapid mass movements have been affecting humans and their goods for a long time; however, people’s responses have changed significantly. Muddy landslides buried a Makah village several times over 500 years, each followed by reoccupation. The death of a woman in 1917, her house washed into Lk Whatcom by a probable debris flow, evidently did not even cause a lawsuit.

Things are different now. More intense use of mountain lands has altered the natural balance of physical forces in some places, while expanding population has taken more people and structures into marginally stable areas. Because debris flows can travel fast and far from their source areas, they can reach out and touch people unaware of any danger.

Since 1970, some spectacularly destructive debris flows have occurred in Washington. They killed at least 10 people, including four each near Marblemount in 1985 and on Bainbridge Island in 1997. Others caused serious damage and some casualties, such as after the Entiat fire (1970, 1 death), during the Greenwater (1977) and Whatcom-Skagit (1983, 1 death) storms, and in the wet winters of 1996-97 and ‘98-99. Information on debris flow characteristics was collected after these and other events, motivated by the need to understand the hazards, to assign monetary responsibility (lawsuits), and ultimately to avoid future death and destruction through changes in policy.

Heavy rain ± snowmelt triggered most of these flows. But usually, part of the cause/blame was allocated to forest roads and logging and/or the presence of people in risky areas, so most policy response came in forestry and land-use planning. Forest-practices rules were amended to address the potential for mass movement due to logging, and related public-safety issues. The Growth Management Act and guidelines directed cities and counties to designate and control development in landslide hazard areas, including those susceptible to debris flows. There have also been changes in other rules, structures, procedures, and employment of geologists and engineers to deal with the issue. As a result, the hazards and risks of debris flows have been inserted more prominently into the public-policy arena. To answer the title queries: we know quite a bit; citizens should care; because lives and money are at stake.