Six reasons the California Mudslides so bad Sure wildfires, drought, and water there's more read this for the real answer.

  When I was young my family lived about 3 miles from the Minnesota River, upstream from where it joins the Mississippi River on its way to the Gulf. There were people living on the "River Bottoms" as we called it, the river is in a river valley, next to the river strawberries were grown, harvested, and sold. I remember going down there with my mom, she had a paper route and I would ride with her. Every spring the river bottoms flooded, after the water receded the people would move back into their mostly brick homes, muck them out, clean everything and start farming again, every year they did that. The last time I visited a few years ago, the roads were closed, there is no longer access to that area of the river. The strawberry farmers are no longer able to farm on the river bottoms, it's a good change.

The Minnesota river today, it's pretty at its normal non-flood
level.

  My dad told me the same thing about California and it's wildfires and mudslides, even though none of us had ever been to that state, he had an opinion. That opinion was the same in the 1960's that he had for the strawberry farmers, every year there are wildfires in mostly the same places. Every year people evacuate, then return to rebuild, but before the reconstruction can begin the mudslides have to do their dastardly deed. But what are people suppose to do? It's so easy to be removed from a catastrophic incident and comment on what people "should" be doing, if you ever want to irritate the devil out of someone utter the words "If I were YOU", well I'm not, and I am fortunate not to be suffering as the Southern Californians are. Actually, most of what happens during our lives is out of our control, natural disasters being a major part of that uncontrollability.
  Southern California has a very diverse topography, ocean facing some of the best beaches in the world are there, next to the beach are cliffs in many places. Passing the cliffs on the beach and crossing the Pacific Coast Highway we encounter high hills, going inland for many miles in places. Then the coastal range of mountains, running from Mexico all along the coast up past the California Northern border and continues northward. People build houses, buildings, and Cities on the slopes, valleys, and canyons that the Mountains create, Lots of brush, weeds, trees, and structures line most of the hillsides and mountainsides.

Photo of Cardiff, "Swamis beach", notice the high cliffs off the
beach, and in the distance the coastal range, rain, gracity, and
steep slopes are contributing factors for mudslides.

Why are the fires and mudslides so extreme, what conditions make this such a volatile area? There are a few things at work here, and they are not new developments:
  * Drought: We had a drought that lasted every bit of 5 years, every living thing in this state was thirsty, the trees suffered badly. Then we had a winter of extremely heavy rain, mudslides did occur during them as well, however the undergrowth was still intact, the soil was not exposed so it did not wash away. Following the drought we had a very robust growing and greening period of about one month, the underbrush dried, then turned golden, (after all it is the "golden state") and the wildfires started. The wind-driven wildfires in the Northern half of the states burned as rapidly as one acre a second, driven by winds gusting to 80 mph. In the North the winds did not last for several weeks, they died down after a week or so, but that's no comfort to the people dealing with that catastrophe. In comparison, the "Thomas Fire" in the Los Angelos area, it started December 4, 2017, it was declared 92% contained and no longer under command of Cal Fire on December 27, 2017, 23 days of uncontrollable winds howling through that huge fire. (Link to Cal Fires report on the Thomas fire)
 282,000 acres were burned, 1,065 structures destroyed and almost 300 damaged. Why was it so huge and intense?
 * Santa Ana Winds: in Southern California there is a wind event that occurs and may last for a month or longer, they can occur any time of the year, although we associate them mainly with the Autumn months. The National Weather Service defines the Santa Ana's as "Strong downslope winds that blow through the mountain passes in southern California, the winds can easily exceed 40 miles per hour. They are warm and dry and are custom made to enhance the effects of wildfires, especially under drought conditions." (Thank you, Wikipedia Link). The fall months in California are the hottest, driest, and most fire-prone months of the year, low humidity, the hot desert winds from inland, (the Santa Ana's), a heated air mass, and high wind speeds all combine to create critical wildfire weather conditions, it's no wonder they are called the "Devil Winds". Further exasperating the conditions are the mountain valleys and passes, they compress the winds through narrow passages causing an increase in velocity.
  * Wildfire: Last year everything in its path burned up, pushed by the extreme winds and the above factors the soil was laid bare by the fires. Hot, fast, and furious the victims in the path of the fires had very little time to evacuate, some did not. After the fires, the soil was laid bare exposing it to the elements, wind, and rain, but that's not the main reason the intensity of the slides are so unbelievable.
 * Topography: Due to tectonic activity on the West Coast mountain slopes are typically steeper, set on bedrock with the loose rock just beneath the soil. The loose, porous rock becomes saturated with water causing it to become waterlogged, then they slip with the assistance of gravity. During periods of intense rain or snow the condition is greatly affected, the underlying rocks, boulders, and sand begin to slip, after complete saturation there is no stopping it. Rocks on the West Coast are millions of years younger than those on the East Coast, time compressed the rock in the East, that and high temperatures from the Earth's mantle have made them less "crumbly" than their Western cousins. Weaker rock, shallow in the earth, and a high water table allow the deluge of water to sit on the rock, allowing water to be soaked up like a sponge, then a slide. Water weighs a lot, one estimate of travel downhill of a mudslide is 20 miles per hour, I don't think the speed matters as much as the weight, and volume of the muck heading downhill. The inertia created by all of that weight is too much to ask of any structure, man-made or natural, to withstand the pressures, it's no wonder mudslides destroy houses.

The inertia caused by water, rock, sediment, and gravel is unbelievable

  * Thicker Sediment: During the last Ice Age, on the East Coast the bedrock did not give way to the Glaciers, they withstood the grinding. On the other Coast however, due to the "Weak Rock", the Glaciers ground away the younger and softer rock causing larger quantities of loose sand and gravel, creating deeper deposits. This sediment buildup is part of the problem with the amount of mud sent down the slopes by allowing the thicker and looser sediment to absorb more water and lubricating the muck on its downhill destructive journey.
  *Earthquakes: The added weight of the water, combined with the sediment, weak rocks, and gravel, the earth-shaking can and does on occasion cause the ground to begin moving. Once all of this begins, there is no stopping it.
  That is a pretty good description of why the intensity and amount of mudflows occur in California, we do see mudslides in other parts of the world, but it doesn't seem to be as intense. I lived in La Quinta California for 3 years, it's in the middle of the Coachella Valley, the great California Desert. While living there I was told a story about a mudslide that occurred in that city during the 1930's. La Quinta is in a Mountain Valley, it's a nice little town, but it's in an enclosed canyon with one way out.

La Quinta, in the 1930's all of those mountains were saturated
with rainwater, then the sediments began to move downhill.

 A heavy rain occurred inundating the entire state with rain, especially so in La Quinta, a hang out of the Hollywood elite in those days. The mud began to flow, the city was buried in mud up to 30 feet deep, everything was buried never to be recovered, some of the structures are still buried. Loose, weak rock, high water tables, a lot of sediment, sand, and gravel combines to create a deadly combination. It was the perfect storm of ingredients combining together to bring us to this spot, everything, wind, drought, wildfire, lots of water, and ground cover eradication, all aimed right at Los Angelos.
  Thank you for reading and sharing, my thoughts and prayers are with the victims of the mudslides tonight, we must always be prepared for a natural, or man-made disaster.

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