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

may be illustrated by a case of a subglacial eruption of Grimsvötn volcano in 1861 when the Skeidara river channel was displaced westward by 13 km [56, 59, 60].

Quite often volcanic eruptions are accompanied by *volcanic-tectonic manifestations*, such as appearance of *fissures* and *deformations of the land surface* (uplift or subsidence of the land surface by a few meters); less common are *collapse calderas*. We mentioned above that during the formation of calderas, not only the river valleys are filled with pyroclasts, but also the formation of flat areas around the calderas, where pyroclastic flows overlap the original relief (**Figure 12**). The abundance of pyroclastic material favors the descent of lahars along the newly formed river network (see **Figure 10**); however, the influence of caldera formation on valleys is not limited to this.

The changes in topography may lead to deformations of valleys' long profiles, to displacement or restructuring of the river network. Earlier we described such changes observed in the Ksudach caldera complex (Kamchatka, Russia); among them are reconfigurations of the valleys' lowermost reaches after the caldera collapse in 1725 (the date is given after [61]), and the stream network restructuring induced by Shtyubel cone growth [29, 30]. The changes in the river network structure accompanying volcanic activities or resulting from them are also catastrophes of various scales; they are often accompanied by outbursts or spillover of dammed lakes and the subsequent descent of mudflows. So, the growth of the Shtyubel cone led to a change in the longitudinal profile of the Lagernyi creek with the formation of a lake, when overflowing and discharging water, a mudflow was formed.

The rapid extrusion growth can also provoke catastrophic processes in the valleys. So, about 1600 years ago [41] because of the giant blocks collapse of the actively grown extrusion Dikiy Greben' (Kuril Lake caldera, Kamchatka, Russia),

#### **Figure 12.**

*The fragments of the pyroclastic plain around Ksudach caldera (white arrows) with Teplaya river valley (Kamchatka, Russia, 2016—See Figure 1, No 12).*

the Ozernaya river valley flowing from the Kuril lake was blocked (**Figure 13**). Lavas large blocks can be traced on the left side of the valley and in the riverbed in its upper reaches. The wreckage is characterized by numerous cracks that arose because of their

#### **Figure 13.**

*Dikyi Greben' extrusion (a - black arrow) and giant blocks (white dotted lines)—Traces of it collapse; b - destroyed dam (white dash line) in the Ozernaya river valley (Kuril Lake caldera, Kamchatka, Russia, 2020—See Figure 1, No 20).*

#### **Figure 14.**

*New dammed lake in the Vulkannaya river upper reaches after the collapse of the Mutnovsky volcano crater wall: Thin white arrow—Direction of rock fragments displacement, thick arrow—The new dam (Kamchatka, Russia, 2021—See Figure 1, No 21).*
