The type of mass movement where materials move like a river is called a mudflow.

Here's why:

* Mudflows are fast-moving flows of water-saturated soil, rock, and debris.

* They are characterized by their fluid-like behavior, similar to a river, as they flow down slopes under the force of gravity.

* Unlike a typical river, which carries mainly water, mudflows carry a significant amount of solid material.

Let me know if you have other questions about mass movements!