In a landslide, masses of rock, earth or debris (rubble or trash) move down a slope. Debris and mud flows are rivers of rock, earth, and other debris soaked with water. They develop when water builds up quickly in the ground, during heavy rainfall or rapid snowmelt and changes the earth into a flowing river of mud or “slurry.” They can occur quickly, striking fast with little or no warning. They also can travel several miles from their source, growing in size as they pick up trees, boulders, cars, and other materials.