The Meaning of Hahamongna

The original settlers of the region were sometimes called the Hahamongna Indians. The word means "Flowing Waters, Fruitful Valley" in the native Tongva language.

Hahamongna News


When Is the Best Time to Prepare for El Niño?





Tim Brick


West Pasadena Residents Association Newsletter


Remember Geraldine, Flip Wilson’s cross-dressing comic character from the 1970s? She had a ready answer for all her peccadillos, “The Devil made me do it.” Now it seems that LA County Flood Control officials are auditioning for a revival of Geraldine, and sensationalistic journalists are letting them get away with it.

Faced with heavy storms that might be coming this winter as a result of the El Niño warming of the Pacific Ocean, the flood bureaucrats keep pointing to Devil’s Gate Dam and the accumulated sediment behind it as the cause for potential flooding along the Arroyo Seco. The journalists, for their part, fail to ask two key questions .Why has the sediment piled up behind Devil's Gate Dam, and why hasn't it been removed to a safe level by now?

Historically sediment from the San Gabriel Mountains flowed through the Arroyo Seco to the Los Angeles River and eventually out to the coast where it replenished beaches and the ocean until 1920 when the County Flood Control District built Devil's Gate Dam. Dams trap sediment, something the Flood Control engineers didn't fully understand in 1920. Since Devils Gate Dam was first built, the accumulation and management of sediment in the reservoir has been a problem.

Flood waters, which rush into the dam basin every twenty years or so, recede and dissipate after the peak, but the sediment stays. By 1938 two and a half million cubic yards of sand and soil had accumulated behind the dam, which has a total capacity of about 7.5 million cubic yards. Since then the sediment level has fluctuated in a range between 2.5 to 4 million cubic yards. After the Station Fire in 2009, a million cubic yards of sand and gravel from the mountains flowed into Hahamongna Watershed Park, bringing the level to its current 3.9 million cubic yards, near the top of the level maintained by County Flood since the 1930s.

The Arroyo Seco Foundation has been advocating a slow, steady sediment removal program for more than two decades. The Flood Control District, however, seems to prefer massive mining and trucking programs occurring every few decades. The last major sediment removal project was in 1994 when County Flood removed 190,000 cubic yards. After the Station Fire, County Flood developed a plan to remove 2.4 million cubic yards in a 3-5 year period of time, 13 times their 1994 project. If the sediment problem is as bad as the flood control engineers say, why haven't they removed any significant amounts of sediment from the basin for over 20 years? The time to prepare for an El Niño storm was ten or twenty years ago.

ASF has joined with Pasadena Audubon Society to challenge the Flood Control District’s Big Dig plan. We seek a sediment management program that will minimize impacts on the habitat at Hahamongna and on local neighborhoods. Meanwhile the Flood Control District has not been able to get the permits they need to comply with applicable environmental law. That's why they won't be removing any significant amount of sediment before the expected El Niño storms.

"Will this winter’s widely-promised ‘Godzilla’ storms be a blessing or a curse? Will they bring relief to California’s long
drought or horrendous flooding throughout our region?”

The storm the Flood Control District projects is a virtual impossibility. Two fifty-year storms (each as large as the largest that have occurred in the last one hundred years) would have to occur within a short period of time, and both would have to be preceded by a thorough burnoff of the upper mountain watershed. The probability of that combination of factors is extremely minimal.

The assertion that the problem is the sediment buildup in Devil's Gate Dam is also fallacious. Devil’s Gate Dam has the capacity to pass floods 50% larger than any we have seen in the last hundred years, but the flood channel through the Arroyo Seco is inadequate to contain the kind of compounded flood flow that the Flood District projects. They have known that for a long time. Four years ago they commissioned a study that identified the properties in peril.

So County Flood built a dam that traps sediment, has not adequately managed the sediment in the basin, and has built an inadequate flood channel. The district is ready to spend $100 million to truck sediment out of Hahamongna Watershed Park to distant gravel pits, but how much have they spent to provide flood protection to the homes, parks and business that they believe are subject to flooding resulting from the overtopping of their channel in the Noah-like flood scenario?

The facilities that County Flood built, including Devil's Gate Dam and the ten mile downstream concrete channel, were experimental, pilot facilities, engineered with a primitive understanding of flood dynamics and of ecosystems. We now have a hundred years of record and a more sophisticated understanding of flood dynamics. A science-based watershed management approach should guide the development of a more sustainable river and flood management program for the next hundred years.

Will this winter's widely-promised “Godzilla” storms be a blessing or a curse? Will they bring relief to California's long drought or horrendous flooding throughout our region? That won’t be clear for a few months, but two things are clear: the devil didn’t make Geraldine do it, and Devil’s Gate is not the source of local flood problems.


Tim Brick, the Managing Director of the Arroyo Seco Foundation, is the former chairman of the Metropolitan Water District of Southern California and the former executive director of the Hahamon