Friday, January 7, 2011

Concrete Ditches, Silt, and Oaks : A Post for the Arcadia Woodlands

Los Angeles is bounded to its north by large, steep mountains.  They are some of the fastest rising mountains on Earth, as described by John McPhee in The Control of Nature.  Add to this equation crumbly rocks and potentially torrential winter rains.  What do you get?  Millions and millions of tons of debris washed out of the mountains after every storm.

When you drive north from downtown Los Angeles, you head upward, slowly.  Much of what you are ascending is the alluvial fans comprised of debris from the San Gabriel Mountains.  I've heard it said that the debris is piled miles deep at the base of these young mountains.  Large washes, as seen in this map from 1880, traversed the valleys, and filled with water and debris during storms.

When the city and its sprawling suburbs were built, there was no room for wandering washes.  Fredrick Law Olmstead, the creator of Central Park, recommended leaving wide spaces for the rivers to wander through during floods.  The rest of the time, they would act as parks and open spaces.  But, instead, the washes were put into concrete ditches.  The concrete ditches couldn't handle the debris and rock from the mountains, so a series of debris basins was constructed to trap it all.

A large, steep, tectonically unstable mountain range sheds a lot of sediment.  It doesn't help the situation, either, when the deep-rooted vegetation is replaced by shallow-rooted invasive weeds, when the fire frequency is greatly increased, or when poorly-designed roads and firebreaks slash across the hillsides and cliffs.  The rate of erosion is increasing, the debris basins keep filling up, and someone has to find something to do with all that debris!

It's a tough call.  There are lots of old gravel quarries nearby (some people apparently WANT debris since that is what the quarries are digging up) and landfills often need fill material as well.  But, residents in the area don't want to listen to trucks driving by, carrying all the debris (even though doing this is the only reason they can continue to live there).  So instead, it is all getting dumped on 11 acres of now-rare alluvial oak woodland next week.

I really don't believe that people couldn't have found a better option than that.

Stories like this make me glad I left California.  It was just too heartbreaking to watch so much of what I cared about and loved being destroyed.  It still is, even from afar.


  1. Naturalist Charlie!! Thank you for your post from afar!

    It's true, this should be a no-brainer, our precious time should be going into resolving - with government - the big issues you allude to here: how to rethink urban LA to account for natural processes and long-term sustainability.

    Really appreciate you making time to write about this.

  2. Thanks! I'm not sure if me posting carries much weight, since I am so far away. Still, it breaks my heart to see this stuff. I still care a lot about the area. One of my early memories of going hiking with my dad in the 1980s involves passing through the Santa Ana Mountains and seeing oaks being bulldozed and uprooted (I think for the toll road?). I remember feeling like there was something so wrong with the world. Unfortunately, it still seems that way.

  3. or i guess i should rephrase. The world is fine. It's some of the people for whom there is something wrong.

  4. My name is Randy. I am a hack naturalist living in Ventura Ca. I found your blog researching beavers in so cal because I saw a beaver today right next to the santa clara river bed less than a mile from the beach. I was very suprised twice today the second suprise was to see that most folk agree there are none in southern cal'

  5. Very interesting! I've never seen any beavers, or sign of beavers, in the Santa Clara River. However, between several people telling me they have seen them in the mountains of Ventura/Santa Barbara counties, a few official sightings, and the historical precedent, there are probably a couple of beaver families in the wildest part of the Sespe watershed (and who knows what else - those canyons, the last refuge of the condors, are amazingly wild). Perhaps the recent heavy rains and high water levels inspired the beaver to travel downstream, since as I'm sure you know much of that river is dry during the dry season.

    I think the beavers could end up being the wild card in the arundo disaster happening in the Santa Clara right now ( Unfortunately, according to a quick google search, beavers don't tend to eat arundo, though I'd imagine they could possibly eat young shoots. An increasing beaver population in the Santa Clara River could result in arundo spreading due to beavers eating cottonwoods. On the other hand, the drastic changes to watersheds by the re-proliferation of beavers could have any number of effects favoring or harming arundo.

    Anyway, a very neat sighting, I'm glad you got to see it! If you see it again, or get pictures, let me know...

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