Debris flows are common in our coastal mountains because heavy rains fall on steep slopes mantled by loose sediments. Highway 99 between Horseshoe Bay and Britannia has a history of destructive debris flows.

1. Torrential rainfall swells streams along the mountain crest.
2. Sediment slumps into a raging stream, forming a slurry (debris flow) that surges down the channel.
3. The debris flow swells in volume as it picks up additional sediment and trees from the channel and canyon walls.
4. The debris flow emerges from the canyon onto a fan where it damages houses, roads, bridges, and a rail line.