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The Hahamongna question

The basin above Pasadena is now a flood threat after filling with debris in the wake of the Station fire. Plants and animals are thriving, but worried people below may end all that.

August 16, 2011|By Sue Horton
  • Fire and water: Willows and sediment fill the arroyo behind Devil's Gate Dam. (Sue Horton / Los Angeles Times)
Fire and water: Willows and sediment fill the arroyo behind Devil's…

As nature goes, the Hahamongna basin is not pristine.

The wide, sandy arroyo, bounded by oak woodlands, sits just north of Devil's Gate Dam on the border of Pasadena, Altadena and La Cañada Flintridge. A gravel operation there, closed decades ago, has left scars on the landscape, and a Frisbee golf course threads in and out of the oaks. Noise from the 210 Freeway on the south end and the Jet Propulsion Laboratory at the north is ever-present. And if all that weren't enough, the Environmental Protection Agency has declared this part of the Arroyo Seco a Superfund site because of groundwater contamination by JPL, which once dumped solvents and rocket fuel in the area.

But what I love about the place is that nature has stubbornly refused to give up. The arroyo provides crucial habitat for stressed flora and fauna struggling to survive along the border between urban land and open space, and every time I go there I see something remarkable. A dense forest of willows has grown up along the seasonal stream bed, and it has drawn an amazing array of wildlife. Once, while walking among the trees, I came upon a fox slowly making its way down the path in front of me. I have seen a bobcat in the stream bed and regularly see deer, coyotes and rattlesnakes. A couple of years ago, workers at JPL were surprised to see a mountain lion sunning itself on a rock near the bridge from their parking lot.

And then there are the birds. This winter, when early rains created a lake in the basin above the dam, I counted six species of ducks, including, for my money, the most beautiful of them all, the hooded merganser. California chaparral species — quail, thrashers, towhees, wrentits and the like — are endemic, and during fall and spring migration a huge variety of expected and unexpected birds put in appearances. The best bird I ever spotted there was a hooded warbler, a small songbird whose usual range doesn't extend west of Texas. It had somehow gotten off course during migration and ended up singing its spring song in a willow tree near the stream bed. I rarely walk in the area without seeing at least two dozen bird species, and often many more.

That may soon change, however.

During the two winters since the Station fire, a huge mass of debris was carried down the arroyo from the San Gabriel Mountains above by a normally anemic stream that swelled after rains to the size of a river. The basin above the dam was littered with hundreds of charred tree trunks, animal carcasses, boulders and mud. So much soil from fire-ravaged hillsides upstream has been deposited in the basin that the bottom of the arroyo has risen as much as 20 feet in places.

It's been fascinating to watch what has happened since. Many native species have come back strong. I've never seen such an abundance of Western toads anywhere as I have this year in the arroyo. And the willows, some of which were buried two-thirds of the way up their trunks, are leafier than ever. Non-native eucalyptus trees, on the other hand, are dying, and a small patch of out-of-place English ivy has vanished.

The question now is what to do about the debris. The county's Department of Public Works has warned that more than a million cubic yards of sediment deposited since the Station fire has blocked one dam outlet and drastically reduced the basin's capacity to contain storm runoff. Heavy rains, the department says, could cause devastating floods south of the dam, including in residential areas near the Rose Bowl and lower arroyo.

Public Works officials initially proposed a massive operation to remove the sediment, which would have also required razing much of the willow forest. The operation would have involved scraping away hundreds of truckloads of sediment a day for months, at a cost of tens of millions of dollars.

But after an outcry from the many locals who hike, jog, picnic and ride bicycles or horses in the area, both the Pasadena City Council and the county Board of Supervisors voted to examine their options more fully. The Public Works Department for now will remove 25,000 cubic yards of sediment from the area closest to the dam. In the meantime, the county will undertake a two-year environmental impact report to look at all the possible actions.

That left one supervisor, Don Knabe, fretting about whether this was enough. He noted, correctly, that homeowners downstream wouldn't be quick to forgive a miscalculation that resulted in widespread flooding.

Of course that would be disastrous. But delaying two years to see if there is a less destructive way to make the basin safe seems worth doing. In the meantime, there is a lot of sediment that can be removed without touching the willow forest.

Ultimately, though, what's happening at the arroyo demands more than an environmental impact report.

California is a place that, as Whittier College biologist Cheryl Swift puts it, experiences "periodic catastrophic disturbances." Fires, floods and droughts have always been a part of the ecology here. What's amazing is not the devastation but the way such a diversity of native species has adapted to it. California willow species, she notes, can be uprooted and knocked down flat, and they just set about sending out new roots and shoots.

Native flora and fauna adapt to Southern California's environment. The humans who live here, not so much. We continue to pave riverbeds, dam streams, landscape with foreign plants, build houses in the path of fires and floods and then wring our hands when confronted with "periodic catastrophic disturbances."

Perhaps, as Swift told me, we should try harder to adapt to nature rather than expecting it to adapt to us.

Sue Horton is Op-Ed editor at The Times.

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