A rapid downhill movement of water, rock, and soil is called a debris flow.

Here's why:

* Debris: This refers to the mixture of materials, including rocks, soil, vegetation, and other debris.

* Flow: This describes the movement, which is typically fast and chaotic, similar to a liquid.

Debris flows are a type of mass wasting, a process where gravity causes material to move downslope. They can be triggered by heavy rainfall, snowmelt, earthquakes, or volcanic eruptions.

Here are some other terms you might encounter that relate to rapid downhill movements:

* Landslide: A more general term for any type of downslope movement of soil and rock.

* Mudslide: A debris flow that is primarily composed of mud.

* Rockfall: The rapid descent of rocks down a slope.

* Slump: A slow, rotational movement of soil and rock.