Landslides often occur in response to heavy or prolonged rainfall. On hillsides, gravity is constantly working to pull the soil and rock downslope, and rainfall infiltrating into the ground changes the forces or stresses acting on those hillside materials. If the changes in stress are large enough to overcome the strength of the hillside materials, a landslide occurs. “Debris flows” are a type of landslide initiated by heavy rainfall or rapid snowmelt; the name “debris flow” is derived from the mixture of coarse sediment that geologists call “debris” and water picked up and carried when they flow downslope or down a stream channel. When they happen, debris flows can move rapidly and travel long distances with tremendous momentum. If they occur where people and things are located they cause damage to buildings, block transportation routes, and sometimes kill or injure people. The USGS landslide scientists are trying to find better ways to determine when rainfall might cause debris flows in order to reduce their danger.