Development within the mountains of coastal British Columbia has recently increased the exposure of people and facilities to debris flows. Attempts to specify weather conditions under which debris flows are apt to occur—such as threshold precipitation—appear not to work because of the highly contingent nature of the flows. Debris must exist in unstable position in or near the channel, and conditions prior to the flow may strongly condition the necessary trigger to mobilize it.

Events have been observed in the following circumstances: locally concentrated rainfall with high antecedent moisture and no snowmelt (the “classical case”); uniformly distributed, moderate rainfall with snowmelt; low intensity rainfall and heavy snow-melt; and heavy rainfall onto deeply frozen, but thawing, ground. A weather-based warning threshold for the British Columbia coast would be fairly complex. At present, such a system would include the substantial probability of issuing nuisance predictions of nonoccurring events.

There is an indication that the incidence of debris flows has increased since 1980. Reasons why this might be so are investigated. Aside from the occurrance of four very wet years since then, no clear meteorologic correlation can be made.