When sediment on a slope becomes saturated with water, the mixture may become too heavy to remain in place, leading to a flow of debris and water as a single-phase unit. This is called a debris flow. Debris flows on Earth often cause significant material destruction and even human casualties, when they occur in built-up areas. During a debris flow, a mixture of stones, gravel, clay and water moves rapidly down a slope. When the sediment subsequently stops, it displays characteristic surface features such as lobate deposits and paired levees along flow channels.