**Abstract**

The river valleys located in volcanic regions are prone to various catastrophic processes, including those catalyzed by eruptions. First, to be mentioned among them are volcanic mudflows known as lahars. They commonly result from melting of ice, snow on the mountaintop, and rainfalls immediately following the eruption. This sequence of catastrophic events—"eruption-volcanic mudflow"—is quite common and has been well studied. When viewed closely the mud and debris flow in the volcanic regions appears to be brought on by various causes, with many factors and agents involved. Quite commonly, an eruption triggers not a single endo- or exogenic event, but a sequence of interrelated catastrophes following one after another. The studied cases allow identifying and describing up to two tens of probable scenarios—successions of catastrophic events in river valleys of the volcanic regions. The specific chain in any particular case depends on volcanic activities and accompanying events, such as seismic shocks, changes in local topography, hydrothermal activity, and erosion. The river valleys and adjoining areas are the most hazardous and vulnerable areas within as much as a few kilometers from the eruption center as the erupted material tends to accumulate in valleys and rapidly transported downstream.

**Keywords:** lava flow, pyroclastic material, gas-hydrothermal manifestations, valley infilling, dammed lake, mudflow, river network restructuring
