Los Angeles County is a beautiful, crowded, complicated place to live, and lots of people want to live there. Southern California has grown tremendously over the last century and in recent years. From 1980 to 1992, Los Angeles County had the largest population growth in the nation. Los Angeles County may be attractive, but it is not a peaceful place to live, in geologic terms. Many population centers are in areas of natural hazards, including earthquakes, floods, landslides, and mudflows.

The effects of urban growth are heightened in a region with such dynamic geology. Increased population puts great pressure on the environment. For example, increased recreation in the mountains increases the number of fires. Fires lead to erosion and erosion can lead to landslides.

The San Gabriel Mountains have always experienced fires, floods, and landslides. But, because more people live in the foothills of the San Gabriel Mountains, in communities like La Crescenta and Montrose, normal geological processes have become hazards. To build these communities, people have altered hillsides and drainage systems, have replaced absorbent top soils with runoff-causing asphalt. As the area has become more crowded, it has become necessary and desirable to build homes into the hillsides. To build safely on steep slopes, people have had to alter the slopes, thus decreasing the slopes' stability.