Southern California is well known for its landslides. Some are large and ongoing, while others have long since ceased movement. Some of the largest landslides in California have occurred in this region. The largest of those, the Blackhawk Slide, is located in the Mojave Desert near Lucerne Valley. During El Nino rainy seasons, slides of all shapes and styles are abundant. This site will concentrate on what causes some of these slides as well as major historic ones.
Significant Landslides:
- Blackhawk Slide
- Portuguese Bend Landslide
- El Nino Slides
- Other Mass Wasting Events
- Landslide Threats in Southern California
This massive landslide, occurring in the Mojave Desert along the northern front of the San Bernardino Mountains, is an unusual landslide. The local geology and topography both seemed to “conspire” to create one of the largest and longest landslides in California history. This landslide occurred about 17-20 kya. The mechanics behind the slide are still being studied. Most likely, the slide was caused by an earthquake of unknown magnitude and allowed to run out so far because of a combination of factors. Those factors, an oversteepened slope and a cushion of air, allowed the rock to slide as far as it did. Still quite visible today, this slide can be seen along State 247 near Old Woman Springs (PM 32.0)
Portuguese Bend Landslide
This rather large ancient slide began was reactivated in the 1950’s during construction of a planned extension of Crenshaw Blvd. Some houses and other structures were destroyed early on, but now damage is mostly confined to the roadways. Utilities have been relocated to above-ground pipes through the landslide area.
El Nino / Rain-induced Slides
These slides occur when heavy rains soak the soils far greater than an average rainy season. Sometimes it takes longer for the water to soak into the soil and reach a potential “slide plane”, so the slide itself may occur weeks or even a few months after a period of heavy rain. Most of these slides occur within days of heavy rains and are small in size. Oversaturated soils on steep slopes are the most common cause of slides. Damage from these can vary greatly, so bear that in mind when building near these locations. The Bluebird Canyon Landslide in Laguna Beach following the 2004/05 winter is a classic example of such slides.
Other Mass Wasting Events
These include rockfalls, debris flows, mud flows, and other events not under the “landslide” category. Any trip through the San Gabriel Mountains will reveal at least a few minor rockfalls on the roadway. The mountain range, in particular, is quite infamous for larger debris flow events as well. In the Montrose area, a series of massive debris flows occurred in on January 1, 1934 which was caused by heavy rains after a massive brush fire. Boulders the size of cars, as well as many cars, houses, and unfortunately some people, were transported by the flow large distances from their origin. Events such as these helped create the need for the debris dams that exist along the base of most of the canyons leading out of the San Gabriel Mountains.