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The situation in California is quite complex. John McFee's excellent "Control of Nature" has a long essay on the interactions between forest fires, geology, and landslides. I'll probably do a shitty job of paraphrasing, but I'll try.

As the population of California has increased, people have increasingly settled farther and farther up the neighboring mountains. The mountain ranges in the area like the San Gabriels are very young geologically. They haven't had the chance to erode down much and are composed of brittle rock just waiting for a chance to break off.

So you have a growing population in an area prone to devastating landslides and debris flow[1].

Meanwhile, the mountains are covered in chapparal. These small scrubby bushes have evolved to endure (and in fact in some species require) periodic wildfires every decade or so. When the chapparal burns, it leaves a coating of ashy dust on the ground. That prevents rain water from soaking into the earth. So when a summer storm comes around later, the water starts sheeting down the mountain, taking dirt, rocks, and boulders with it.

This is a devastating event. It's very hard to build defenses against it, and hard to predict precisely when it's going to occur. You're talking going from "everything is fine" to "thousands of tons of rock crashing through homes" in a matter of minutes.

Ironically, debris flow would be easier to predict with controlled burns, but it's pretty hard sell to say, "Yeah, you're house is going to get flattened next month and we're going to make it happen." So, because of the much greater threat to life (people die in big debris flows all the time) and property, California ends up very hesitant to do controlled burns of the chapparal.

[1]: https://en.wikipedia.org/wiki/Debris_flow




Thank you. I've added that book to my wishlist, it sounds really interesting. Don't really ever have to worry about landslides in Florida (just hurricanes and sinkholes).


Wouldnt, also, in certain cases, the ash mix with the soil and make a more hardened clay-like mix which would harden and prevent a certain amount of erosion?


Not in California where the mountains are growing at a fast rate (for a geological process fast). The roots hold the mountain up while the trees grow, but when the trees burn those roots no longer have life and the next rainstorm lets the mountain side fall as the mountain has grown.

Read the book, it is described much better than I can summarize.




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