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07/01/1996 07:00:00

Question of the month: Why is there so much gravel in the San Gabriel Valley, especially around Irwindale?

Question of the Month Answered by: Lee Silver, W. M. Keck Foundation Professor for Resource Geology

The San Gabriel Valley contains huge amounts of gravel because the San Gabriel River carries broken rock out of the nearby San Gabriel Mountains. The San Gabriels produce especially large amounts of gravel for several reasons.

For one thing, they are very steep. The San Gabriel River leaves the mountains at the Santa Fe Dam, where the elevation is roughly 1,000 feet. Within just a few miles of this spot the river reaches mountains in the 7,000- to 10,000-foot range. This steepness, or "gradient" as geologists call it, affects how much mud, gravel, and rock a river is able to carry. The higher the gradient (the steeper the slope), the more material a river can transport.

The reason the gravel accumulates mainly in Irwindale is that the gradient decreases suddenly—that is, the river flattens out—right there. The river can carry lots of debris as long as its course is steep, but as soon as the river leaves the mountains and its channel levels out, most of the debris settles out. Rock and gravel have been accumulating in the San Gabriel Valley for hundreds of thousands of years and is probably thousands of feet deep in the Irwindale area.

Another factor that helps the San Gabriels produce more gravel than most other mountains is the ongoing earth movement we have in Southern California, which we sometimes feel as earthquakes. When the earth moves in a large earthquake, the mountains often grow a few inches, and sometimes gain several feet. So even though erosion is constantly wearing down mountains everywhere, the San Gabriels are better able to maintain their height, steepness, and ability to produce lots of gravel.

Also, because Southern California sits astride a boundary between tectonic plates, the rock in the San Gabriels tends to be highly fractured. This makes it treacherous for climbing, and means that it breaks into chunks more easily than solid, unfractured rock.

Because of their height, the mountains can create their own weather, and actually draw moisture out of passing clouds. The high elevations get many times more rainfall than the flatlands just a few miles away. This heavy rainfall collects in the steep streams and rivers and forcibly sweeps the gravel out into the valley.

The location of the many gravel quarries in Irwindale is a historical accident. Homeowners didn't want to live on the San Gabriel River floodplain because of the risk of flooding. So the gravel companies moved in instead, established mineral rights to the land, and started mining gravel. The gravel is used as an aggregate material in concrete, and is one of the Los Angeles region's most valuable mineral products.

Besides gravel, the San Gabriel River also carries a lot of water out of the mountains. Much of the water is left to stand in settling basins, which can be seen to the west of the 605 Freeway. They may look like flooded gravel quarries, but actually the water is left there on purpose, so that it can soak into the ground and replenish the underground water table.