Debris flows is a kind of mass wasting event which can occur suddenly and inundate entire towns in a matter of minutes. As the name suggests, debris are made up of debris which may include small mud particles to as large boulders, trees, cars, and parts of buildings. The phenomenon is fueled by heavy rain which washes material from a slope or when it sheets off of a freshly burned stretch of land. It then acts off as a lubricant for the debris so much so as the rapidly moving water cascades down the slopes, and into the canyons and valleys below. As the system gradually picks up speed, the flow takes on the characteristics of a basic river system. The faster the water flow, the more the water can pick up. As the water picks up more mud and rocks, it begins to resemble a fast "flowing river of concrete".
There have been many measures undertaken to try to control the debris flow and McPhee includes them in the story. Some of them are measures that are reactive such as building barricades or deflection walls out of sandbags or concrete. These methods sometimes are effective and can redirect the flow to prevent the damage to the property but then again, one can never be sure how long they will last with the uncertainty associated with the momentum of the debris flow. There is also the question of where it gets redirected.