Erosion and sedimentation are natural processes driven by wind, water, and gravity. Over time, these processes shape landscapes, carving valleys, transporting sediment, and influencing river channels. While human activities can alter the rate and pattern of erosion, the movement of soil and sediment remains a constant factor in the changing earth.

Although Southern California is known for its mild climate, the region is prone to severe flooding. Pacific storms, typically occurring from October through March, encounter the steep mountain ranges surrounding Malibu, the Los Angeles Basin, and the Antelope, San Fernando, San Gabriel, and Santa Clarita Valleys. These storms often generate large amounts of runoff that flow onto the flatlands and toward the Pacific Ocean or Mojave Desert. If wildfires have occurred during the dry season, runoff increases in volume and carries higher concentrations of debris. Because the mountains’ geological composition erodes easily, stormwaters frequently transport a heavy mix of sediment, rocks, and vegetation downstream.

Since the severe mudflows and loss of life in La Crescenta and Montrose during the 1934 New Year’s Day Storm, erosion has become an increasingly significant concern as development has expanded in the valleys and mountains of Los Angeles County. Homes and businesses now line the rivers and streams that carry storm flows to the ocean and desert, while residential areas extend across hillsides from Malibu to Claremont and from Santa Clarita to La Cañada Flintridge. Erosion is a critical factor in the design of channels, drains, and their upstream components. Debris retention structures, including Public Works’ debris basins, are designed to collect eroded material (sediment) and prevent damage to downstream properties and infrastructure. Large volumes of sediment also accumulate in Public Works’ reservoirs. Without ongoing maintenance, debris basins and reservoirs could become ineffective, leaving downstream areas vulnerable to flooding and property damage.

The sediment collected in Public Works’ reservoirs and debris basins—material that would have otherwise been carried to the ocean—must now be removed through other means. One common method is trucking the sediment away. Depending on the frequency and intensity of winter storms, reservoir clean-outs may be repeated several times over a decade. Smaller debris basins can be cleaned multiple times within a single storm season.