<em>Lahar</em> is the name given to this specific type of mass wasting.
Explanation:
Lahar can be described as a mixture of hot and cold mud or debris flow that flows from the slopes of a volcano and is most likely to enter and pollute a river. A lahar can move very fastly from the slopes to the river flow. Lahar's can be extremely destructive depending on the speed at which it rushes downstream.
The scenario mentioned in the question during which an eruption of Mount St. Helens rushed down the Toutle River is an example of lahar.
