Recurring landslide and mudflow events in the Wrightwood area of Southern California are parts of a composite cycle of landslide activity that includes three recognizable stages. The three stages are interdependent, occur in sequence, and are of different duration. Deposits of the first stage--the largest in size--are removed to positions further downstream by the activity of second- and third-stage landslides.
First-stage landslides are represented by huge slumped masses derived from steep bedrock slopes in the canyon heads; the material moves down the principal stream drainage, which may be completely filled with debris. Second-stage activity develops as streams cut a network of branching channels into the massive first-stage deposit. The second-stage landslides are chiefly slumps from the older slide mass and from adjacent bedrock slopes. The movement of these slides generally is downslope toward actively eroding drainages. Third-stage activity includes mudflows that accompany the spring melting of snowpack. The debris moves down the stream channels to depositional reaches on major fans. Removal of sufficient amounts of the first-stage landslide mass to the fan by second- and third-stage events resets the bedrock slope of the main drainage for another first-stage event.
The first-stage landslides in the Wrightwood area are of prehistoric origin, and their recurrence interval in any one canyon is probably several thousand years. The active duration of a first-stage landslide is one to several thousand years. Second-stage landslides last one to several years and are apparently preceded and triggered by a series of high-precipitation winters. The duration of observed third-stage (spring mudflow) sequences ranges from a few days to as much as six weeks; peak mudflow activity apparently results when a-heavy spring snowmelt occurs during a period of second-stage landslide activity.
