Mud flow, debris flow…what’s the difference?

Mike Lawler

I’ve found myself explaining to my “flatlander” friends the dramatic difference between a mud flow – which is what we mostly saw last weekend – and a debris flow, which caused the large-scale destruction of lives and property in 1934.

I think most of us understand the ground we’ve built our homes on. We live on layer after layer of alluvial fan. The San Gabriel Mountains are some of the fastest growing mountains in the world. They are made of brittle, decomposing granite, which fractures with each earthquake like safety glass; all those chunks of granite fall down into the canyons, along with the sand from the decomposing rocks. When it rains hard, those chunks of rock and sand are carried out across the valley, fanning out from the canyon mouths – thus the term “alluvial fan.”

In a rainstorm, the faster the water comes down the mountain, the more rocks it can carry. Our newly denuded mountains ensure that the water, with no foliage and roots to slow its downhill flow, will be moving very fast by the time it reaches the rock-filled canyons.

A little water in the canyons carries mud, the water being strong enough to push only fine grains of sand. Larger volumes of water can move small stones, then larger rocks, then boulders.

It’s at this point where we find our fundamental difference between “mud flow” and “debris flow.” As the amount of particulates increase in the flow of water, and the speed increases, there comes a tipping point where the physics of the flow changes completely. The larger rocks begin to “float” on the material below it, allowing them to pick up speed, and in some cases surf the front of the flow. This creates a churning wall of mud and boulders that can move quite fast – 30, 40, or 50 miles per hour. It’s that churning mass that tore some bodies into pieces back in that big flood in 1934.

The mass, which geologists call a “slug,” can’t be stopped by anything man-made, which is why in photos of the ’34 flood you see bare foundations from which houses were sheared off, as though by a gigantic bulldozer. The debris flow turns back into a mud flow only when it slows down enough for the rocks to drop out, while the mud and water continue on.

One of the purposes of debris basins is to serve as a speed bump for debris slugs. Once the slug hits the basin, it pauses long enough for the rocks to drop out, and the water and mud can continue down the flood control channel. If the basin is full, the slug doesn’t stop, and it overwhelms the channel below, fanning out across the valley like it has for hundreds of thousands of years.

What we had this weekend was mostly mud flow. In some cases, the water wasn’t strong or fast enough to pick up the boulders and make that transformation to a debris flow; in other cases, the debris flow coming down the canyon slowed enough to allow the larger material to drop out. Certainly there was plenty of debris there, but it was minor compared to a full-fledged debris flow.

We did have a lot of destruction last weekend and I am not diminishing the impact of that, but it could have been much, much worse. The phenomenon of debris flow is very unpredictable. It has happened on a huge scale in pre-history (that’s where the rocks in your yard came from), and we know it will happen again.

We’ve built our homes in harm’s way. We can’t predict major debris flows, but we can say when they are more likely. L.A. County and Glendale have been very clear with residents here. Debris flows can’t be predicted and can’t be fully controlled. We just have to get out of the way.

Mike Lawler is the president of the Historical Society of the Crescenta Valley.  He can be reached at