Hundreds of cars were stranded when a mudslide swamped a Southern California freeway last night in what felt like a chilling dress rehearsal for the El Niño on the way. The mudslide is a reminder that it’s not just heavy rain we need to worry about—heavy rain falling on the state’s parched ground will bring disaster.

Yesterday afternoon a powerful storm dropped up to six inches of rain on the region causing dangerous flash flooding. In two separate incidents, a hillside slid onto the busy 5 Freeway over a mountainous pass called the Grapevine and hundreds of cars were trapped on a smaller highway in the High Desert.

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Ground that’s exceptionally dry becomes crusty and hard, preventing water from naturally infiltrating the soil. When rainfall does come along, the ground can’t absorb water as well, so the rain flushes it away instead. The ground is therefore much more likely to become unstable.

So in both cases yesterday, it wasn’t just water that made life difficult for drivers and residents, it was the ground coming with it in the form of thick sludgy mud. As KCAL 9 reported, in some instances cars were engulfed in up to six feet of mud.

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Remember those NASA drought maps that specifically track soil moisture? The areas of the West with the driest soil are pretty much the same areas which are predicted to see heavier than normal rain due to El Niño.

We saw what happens when drought-baked ground and heavy rain meet to dramatic effect in July, when rare summer storms washed out a freeway bridge just east of Palm Springs.

Fires—and it’s been a record-setting year for blazes in the West—make this situation even worse, as there’s dry soil, less living vegetation to anchor the soil in place, plus debris from dead or burned trees which can make flows even more dangerous.

We’ll see El Niño sculpting new landscapes throughout California, and urban areas are particularly poorly equipped to manage large amounts of water. So if in fact El Niño might send enough rain to “end the drought,” the truth is, most of it will get flushed out to sea through our storm drains. Los Angeles has just set up a water reclamation system where the LA River hits the Pacific that will hopefully be able to catch, clean, and put lot of that runoff to work. But it won’t be able to recycle a wall of mud.

Follow the author at @awalkerinLA

Top image via KCAL 9