By Charly SHELTON

California needs rain but for the areas recently burned by wildfires rain can mean mudslides. And mudslides mean evacuations. The Crescenta Valley is no stranger to mudslides and various other debris flows, as was seen in the aftermath of the 2009 Station Fire and, of course, the New Year’s Flood of 1934, which sent buildings floating away down the hill. But the likelihood this year of major mudslides in the Crescenta Valley is minimal. That doesn’t mean, though, that heavy rains are without danger.

First, we need to know about the geology of the area. With work set to start at the former Verdugo Hills Golf Course, a recent Environmental Impact Report was prepared to give an overview of the area – both naturally and as it relates to human use from the past and present and in the future. The report describes the geology of the Crescenta Valley area as squished between several smallish faults to the north, east and south, and sitting on a thrust fault block, or a little peg of earth, that just gets pushed forward in an earthquake rather than shaken apart.

“[The Crescenta Valley is] near the northern edge of the Los Angeles Basin [and bordered on the south side by] the northwestern flank of the Verdugo Mountains,” the report said of the overall project area. “These mountains are a portion of the Transverse Ranges geomorphic province of Southern California. The Transverse Ranges are characterized by east-west trending geologic structures, as opposed to prevailing northwest-southeast structural trends that dominate elsewhere in the state. The Verdugo Mountains are characterized primarily of Mesozoic or older age crystalline metamorphic and igneous basement rocks.”

Simply stated, what that means is there is a reason the town is colloquially referred to as Rock-rescenta. Any resident knows that about one shovel’s depth into their yard is a pile of rocks that comprise the ground the town is built upon. These “Mesozoic or older age crystalline metamorphic and igneous basement rocks” are predominantly granite from the Cretaceous period, when T. rex and Triceratops walked the Earth in other areas of the country while much of California was still underwater. That’s at the basement level; then strewn on top of that is “Quaternary age alluvial deposits, including stream terrace deposits and older alluvium, as well as recent alluvium occur in present-day stream courses,” the report said. Also known as river stones.

So the geology of the Crescenta Valley is a big chunk of granite that formed under the ocean during dinosaur times, which was then pushed up to the surface between 5 million and 1.6 million years ago, and has had rivers depositing hefty loads of dirt and river stones on top of it as the peaks of the mountains slowly eroded away and fell into the river. And about 10,000 years ago – a blink of an eye in geologic time – humans settled in the area and established villages.

When a rainstorm does come, we don’t have much aside from rocks and a bit of dirt to slough off and create a mudslide. What little we do have is held in place by the plant life that grows everywhere in the mountains. After the Station Fire, there was hardly any plant life at all and so the burned plants, the soil and some of the smaller rocks got swept up and washed away in the rainflow. This built up and carried away bigger rocks, turning into a mudslide. But since the winter of 2010, there hasn’t been a major problem with this. Flooding is what we have to be aware of every year during the rains.

With rocks as the majority of the local ground area, the rainwater isn’t absorbed as readily as it would be somewhere in the Great Plains or northeast that have a deeper soil climate and fewer large rocks at the surface. That’s not to say rain doesn’t soak in or that this is the worst condition for taking in water – it’s easier to absorb rain into the natural, rocky ground than it is to absorb through the pavement that covers most of the LA area – but this is a limiting factor.

So while the danger of mudslides washing away homes in a debris flow may be minimal this year compared to years past, if a good torrential downpour finds its way to the Crescenta Valley there could be some wet garages and flooded living rooms if the proper precautions aren’t taken.

Sandbagging any doors that may not be watertight – like garage doors or front doors – may be wise if a major storm is expected. And making sure to clear all of the drains in and around a home while the weather is dry will keep them from backing up. This will prevent having to clear them when they are elbow-deep in rainwater as a pool edges its way toward front doors.

  Charly Shelton