California, USA (1977) (submitted by Anonymous; added on 8/20/2000)
The San Gabriel Mountains in Los Angeles, California are relatively young and still forming. LA is famous for earthquakes. The mountains are famous for mud slides. It is logical, because the San Gabriel Mountains are full of the same faults or cracks that cause California’s famous earth tremors. During the dry seasons of the area, fire sweeps across the slopes, leaving them bare of trees and shrubbery. As a result, nothing remains to hold the soil layer in place. Heavy rainfall after fires can cause tons of mud, rock, and debris sliding downhill so fast that the flow outruns moving cars. It doesn’t often rain in southern California, but when it does, it seems to descend all at once. In 1977, a foot of rain fell in 24 hours, creating a wave of debris 20 feet high. As it moved along at 550 feet per minute, the thick mixture of water and dirt resembled wet concrete. Huge trees are sometimes carried along the mudflow, in addition to strange objects like picnic tables, propane tanks, house trailers, cars, trucks, and people. More than 600,000 tons of debris filled houses from floor to ceiling or crushed them in passing. In some landslides, people have been buried as they slept. To accommodate this problem, Los Angeles has built 120 bowl-shaped pits to catch some of the flow. Resembling football stadiums, these debris basins catch the flows of mud and earth, which is then carried away by trucks. So far, they have trapped about 20 million tons of the San Gabriel Mountains. Cleaning them out costs more than $60 million per year. Scientists who study this phenomenon claim that 300 to 400 of the 10,000 houses in this region will be destroyed in ten years. However, people keep moving to the cool, clean air and breathtaking views on the slopes of the San Gabriel Mountains.