Sea to Sky Drag the yellow box in the small image at the lower right to scroll the big image. Debris flows are common in our coastal mountains because heavy rains fall on steep slopes mantled by loose sediments.
Torrential rainfall swells streams along the mountain crest.
The debris flow swells in volume as it picks up additional sediment and trees from the channel and canyon walls. The debris flow emerges from the canyon onto a fan where it damages houses, roads, bridges, and a rail line.