SanGabrielMountainsFlood

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= 1938 Los Angeles Basin and San Gabriel Mountains Flood =

On March 3, 1938, the Los Angeles Times printed a drawing of the Los Angeles basin and the San Gabriel Mountains and titled it “Where Down Pour Has Stored Vast Lakes of Water”. In it, there are various areas noted for having overloaded reservoirs of water: Hollywood Dam, Big Tujunga Dam, Haines Canyon Debris Basin, Big Santa Anita Dam, along with several others.  This flood was considered a 50-year flood,because it has a 2 percent chance of occurring yearly. This image pertains to a massive rainstorm that hit the Los Angeles region in the last days of February and the first days of March in 1938. In the years from 1882 to 1978, the overall average rainfall for Pasadena was 19.87 inches. From 1934-35 to 1937-38, the average rainfall in Pasadena was 25.79 inches. During 1937-38, in the year of this storm, the yearly total of rain for the area was 31.38 inches. (Fig A) Four and a half inches of the 31.38 inches that fell during the 1937-38 season fell on the Los Angeles area during the first three days of the storm, from February 27 to March 1, 1938. After a fifteen-hour respite the rain started up again and by March 3, 1938, six more inches of rain had fallen. This meant that the five-day total rainfall for the storm was more than ten inches, thirty percent of the total for the year (a total that was already about 157% higher than average). At this point, what the Los Angeles Times called the “killer flood of the century” was unleashed.

Depending on which source you are referring to, this storm resulted in the deaths of anywhere from 115 to 177 people. All the communities and cities in the Los Angeles region were inundated with water, some more severely than others. Those communities backed up against the south side of the San Gabriel Mountains were hard hit (Photo 1). The Rose Bowl (Photo 2), in Pasadena, survived, but close to half of the golf course next to it was destroyed. Neighborhoods running the length of the Los Angeles River down into Long Beach were also affected. Hundreds of people were forced to flee their homes. The Santa Ana River in Orange County flooded the surrounding community, sending fifteen people to their deaths. Back towards the San Gabriels, the gates of the Big Tujunga Dam were opened to prevent damage to the facility, delivering more flood waters into the community. Flooded streets cut off movie studios. Railroad lines and the Pacific Electric Red Cars were closed down. 1,500 homes in the Los Angeles area were destroyed. The Acadamy Awards ceremony had to be delayed for a week. Estimated losses for Los Angeles County alone came to about six million dollars.



Area Geology
Above Pasadena (located northeast of downtown Los Angeles at the base of the San Gabriel mountains) two large canyons, the Arroyo Seco (Photo 3) and Eaton Canyon, drain into the city from the San Gabriel mountains. Downstream, near central Los Angeles, the Arroyo Seco meets with the Los Angeles River. During the flood, run off from the San Gabriels swelled the banks of the Arroyo Seco (Photo 3) and Los Angeles rivers and flushed water all the way down to the Long Beach harbor. The San Gabriels are a series of mountains that border the Los Angeles basin to the north. Besides Eaton Canyon and the Arroyo Seco, the mountains also have the Santa Anita, San Gabriel, San Dimas, San Antonio and Dalton canyons. The mountains are pushed up by the San Andreas fault in the north, by the thrust and reverse faults of the Cucamonga-Sierra Madre zone in the south and southwest and by the San Jacinto fault in the east. During a rain, it is common for the San Gabriels to hold up the movement of clouds coming in from the Pacific. These conditions of upthrust, canyons, water flow and weather, combined with the fact of it being an El Nino season, conspired to create the circumstances that lead to this storm.



Flood Control
During the time of the flood Los Angeles had spent $50,000,000 on flood control and built fourteen flood control dams. The “killer flood of the century” none-the-less served as a reminder that more flood control work needed to be done. The Hanson Dam in Tujunga was incomplete, resulting in the canyon unloading tens of thousands of cubic feet of water per second into the streets. Much of this overflow made it into the L.A. River, destroying much of the new construction work that was underway there by the United States Engineers. Some of the damage due to the incompletion of the Hanson Dam was mitigated by the holding power of the Big Tujunga Dam, (whose water was eventually forced to be released, illustrating the need for completion of the Hansen Dam). The effect of all this activity was for the U.S. Congress to pass the Flood Control Act of 1941. Part of the flood containment program thus enacted involved building debris basins at canyon mouths where the ravines emptied their contents from the mountains above. (Today there are over 678 drainage basins in the area). Other actions taken involved the building of large flood control basins and the deepening and widening of stream basins. (During the flood, when the Devil’s Gate Dam above Pasadena begin to loose water over it’s spillway, the channels beneath it were unable to handle the excess. The Arroyo Seco channel, which cost $40,000 to install in the first half of the 20th century, was one of many that needed to be enlarged and reconstructed.) Further flood containment projects that took place after the 1938 flood involved the completion of both the Sepulveda Dam and the Whittier Narrows Dam.

**Conclusion** A Los Angeles Times article titled “Small Losses Prove Value of Dam System” on March 4, 1938 claims that the destruction and devastation of the 1938 flood could have been much worse if Los Angeles county hadn’t had the flood control system in place that it did. Much of the damage done during the 1938 flood was done to flood control projects that were in the middle of being constructed. It’s important to note that while no flood control system that the tax payer can afford may absolutely safeguard against all heavy storms, it’s evident that flood control precautions do serve to thwart damages. Flood control should be a continued matter of discussion in this era of global warming.