*Catastrophic Processes in River Valleys of Volcanic Regions: Geomorphologist's Point of View DOI: http://dx.doi.org/10.5772/intechopen.108141*

fall at more than 2 km. According to our estimates, the dam height reached 20 m and its destruction was inevitably accompanied by a debris flow.

Among the factors contributing to the hazardous processes in river valleys are the presence of unstable rocks (fissured, loose, or altered by hydrothermal processes), seismic shocks (even weak), or extremely heavy rainfall. There are well-known facts of debris avalanches, rockfalls, and other kinds of active mass wasting not only during the eruptions (as in the case of Shiveluch and St. Helens), but also long after them (as on volcanoes Parinacota in Chile, Shasta in California, Casita in Nicaragua, and others) [2]. Obviously, seismic events, both accompanying volcanism and directly independent of it, can cause the destruction of *the crater lake walls* or the body of a volcanic dam of various origins [53, 62]. In any case, these will lead to lake outbursts and mudflow descent. On the other hand, if an earthquake triggers a landslide, a new dammed lake may appear in the valley. In August 2021, we observed the consequences of a collapse on the northern wall of the crater of the Mutnovsky volcano (Kamchatka), where 19.08 another dammed lake appeared and mudflow descended (**Figure 14**).

Large-scale geomorphic catastrophes may occur in valleys within the zones of hydrothermal activities. In those zones, rocks in the valley sides are densely fissured, essentially altered, and often turned into clay by the chemical weathering processes and water encroaching. The rocks' hydrothermal weathering and moistening processes stimulate a wide range of slope processes—rockfalls, landslides, slumps, flow slides. The hydrothermal clays distribution and the presence of the steam and thermal water outlets on the valley slopes favor the numerous displacements, resulting in the formation of the local multilevel landslide and block-slide terraces. In some cases, several tiers of similar terraces can be observed (**Figure 15**).

In such areas, there is a significant widening of the valleys due to active slopes flattening. At the same time, in the bottoms, there is an accumulation of the slope material, displaced because of landslides and collapses with the periodic blocking of the valleys and the formation of the temporary dams and the dammed reservoirs, in which the river sediments accumulate. The dams' length can reach 500–700 m along the valley. The further slope and alluvial material transportation and redeposition occur mainly due to the debris flows, which are formed either directly during the gravity collapse, or during the dams' destruction and the descent of temporarily dammed reservoirs. This is well confirmed by the observations carried out in the

#### **Figure 15.**

*Landslide terraces (white arrows and dashed lines): A - on the Geysernaya river valley sides (Uzon-Geysernaya caldera, Kamchatka, Russia—See Figure 1, No 25), periodically blocking the river with subsequent debris flows originating (2013), b - in Kislyi Creek, Mendeleev volcano, Kunashir Isl. (Russia, 2018 - see Figure 1, No 3).*

#### **Figure 16.**

*The Geyzernaya river valley (Uzon-Geysernaya caldera, Kamchatka, Russia, 2021—See Figure 1, No 25): A - partly destroyed dam 2007 (1) with drained lake (2), b - dam 2014 (white arrow). Black arrows—River flow direction, white dashed lines—Dam borders.*

#### **Figure 17.**

*The debris flow deposits formed the accumulative terrace (white arrow) in the Geysernaya river mouth (Uzon-Geysernaya caldera, Kamchatka, Russia, 2021—See Figure 1, No 25): 1 - Geysernaya river, 2 - Shumnaya river.*

Geysernaya river valley (Uzon-Geysernaya caldera, Kamchatka, Russia) [63–67], where currently there are two similar dams—one (2007) is already cut by the river, and the other (2014) is in the early stages of erosion (**Figure 16**).

The debris flow deposits form the accumulative terraces (**Figure 17**); in the valleys of some watercourses, the debris flow embankments with a length of a few hundred meters remain, which sometimes undergo cementation. During mudflow material splashes, the recorded heights of which can reach 40 m [68], the mudflow material covers with a thickness of 0.5–1.0 m to 3–5 m remain on the valley sides and its

terraces. Similar debris and mudflow traces are typical for other watercourses of the high hydrothermal activity territories in the Kuril-Kamchatka region [69, 70].
