**3. Conclusion**

River valleys in volcanic regions become occasionally zones of rapid deposition of juvenile and resurgent material. The latter comes from volcanic eruptions of effusive or explosive type, volcanic-tectonic or hydrothermal processes (lava outflow, pyroclast ejection, volcanic cone breakdown, or explosion, sliding of rock mass weakened because of hydrothermal activity). That results in the river valleys being filled with volcanic material diversified in composition and properties, with the deposition proceeding from the upper links of the river network downstream. At the same time, the valleys serve as routes of active, and mostly intermittent (steplike), displacement of the material by various agents. Most often it takes place as a result of volcanic mudflow descent immediately after eruption due to ice or snow melting or resulting from abundant rainfalls. In some cases, however, a series of 2–3 catastrophic events occurring in a valley ends with a mudflow descent; the mudflows may occur repeatedly and are known as secondary lahars. As follows from long-term observations, the series of such events may continue through a few decades, or even centuries [2].

When considering geomorphic hazards controlled by volcanic activities, we recognized a lot of kinds of event chains of that type. An analysis of the chains permitted to identify the main geomorphic factors responsible for catastrophic events in river valleys are as follows: quick depositions of volcanic products; displacement of loose rock masses; slope angle changes; and accelerated erosion. Mudflows moving loose material downstream "in the pulse mode" occur when the loose deposits brought into valleys are actively eroded by the streams arising not only from abundant rainfall, or snow and ice melting but also resulting from an abrupt release of water from dammed lakes in case of the dam breakthrough. We agree with S. Chernomorets and I. Seinova [11] in that the mudflows are "usually the final stage in the chain of catastrophic events in the course of eruptions" (p. 57). There are also many other exogenic—fluvial, eolian, cryogenic processes taking part in the processing and transportation of the volcanic deposits, but it is much rarer that they assume the form of catastrophes [45].

It may be concluded from the above that the rivers in volcanic areas are particularly vulnerable and endangered because of several factors: large volumes of the material supplied and transported; high rates of endo- and exogenic processes on the adjoining territories; and highly energetic activity of the streams themselves, which induces a wide assortment of hazardous events and exerts a notable effect on the relief-forming processes.
