Final answer:
The downslope movement of a debris flow is primarily dominated by rock and mud.
Step-by-step explanation:
The downslope movement of a debris flow is primarily dominated by rock and mud. When slope materials become saturated with water, a debris flow or mud flow can occur. This slurry of rock and mud can pick up various objects such as trees, houses, and cars, leading to the blocking of bridges and tributaries and causing flooding along its path.