Landslides occur often in dry or semi-arid climates in areas with steep slopes or mountains.
The California coastline, with its steep cliffs and years of drought punctuated by seasons
of abundant rainfall, is more prone to landslides than many other regions. In areas where
landslides are a frequent hazard, communities have put together warning systems, to help
people be better prepared. Around San Francisco Bay, the National Weather Service and
the United States Geological Survey have a set of rain gauges that monitor the condition of
the soil. If soil becomes saturated, the weather service will issue a warning. Earthquakes,
which can happen along western California’s abundant faults, can also trigger landslides.
Mudflows and Lahars
Mudflowsandlaharsare also dramatic and dangerous natural hazards produced by the
force of gravity (Figure10.52). Mudflows tend to follow existing stream channels or ravines.
Mudflows often occur on hillsides with soils rich in clay and with little sand or gravel. Where
there is little rain, there is not much vegetation to hold the soil. That means mud will flow
when a large storm produces a lot of rain in a short time. The saturated soils, without plant
roots to keep them in place, flow downhill, following river channels, washing out bridges,
trees and homes that are in their path.
Figure 10.52: The white areas on the otherwise green mountainsides mark scars from nu-
merous mudflows. Mud deposited by the flow can be seen along the river channels. ( 27 )
Some mudflows are as small as a few meters in length, width and depth. Others can travel for
thousands of meters, moving materials tens of meters deep and hundreds of meters wide. On
steep slopes, a mudflow might travel very quickly, ending abruptly when it reaches flatter
ground. Thicker, more viscous mudflows move over a period of days or even years. The
movement could be as slow as several millimeters/day or perhaps several meters/day.
A lahar is a particular type of mudflow that flows down the steep sides of a stratovolcano
(Figure10.53). These explosive volcanoes produce tremendous quantities of ash and dust
as they erupt. Snow and ice from the top of the volcano melt, producing floods of meltwater.