**2. Chains of catastrophic events in river valleys of volcanic regions**

The river valleys in volcanic regions often originate on slopes of volcanic cones. In their uppermost reaches, they have a look of erosional hollows ("barrancos" in Spanish). If a river flows in a fault zone with eruption centers of its own (moderatesize shield volcanoes), the latter appears to be within the valley itself. Such phenomena may be seen, for example, in the upper reaches of the Bolshoy and Maly Yenisey, in the Jom-Bolok (Zhom-Bolok) river valley in the Eastern Sayan mountains (Southern Baikal volcanic region, Russia) [15–17], in the Bolshoy Anyuy river valley in Chukotka (the Northeast of Russia—see **Figure 1**) [18].

In either case, an *effusive eruption* may result in the river valley being filled with lava flow, sometimes over a length of tens and even more than one hundred kilometers. The basalt flows of the Middle Pleistocene age—among them Undara (Queensland, Australia) and Pampas Onduladas (Mendoza, Argentina), both up to 160–170 km long are recognized as the longest-**Figure 1**) [19, 20]. There is a lava flow which is greater in size in the Maly Yenisey river valley (Eastern Sayan mountains, Russia)—it is 175 km long, up to 1.5 km wide, and its volume is estimated at 40–50 km3 [16]. The youngest basalt flow comparable to the above-named in length (~140 km) is Thjorsa (Þjorsa, Iceland) [21] dated to the Holocene. Another lava flow of young age (about 13 ka BP) is the Jom-Bolok (Eastern Sayan mountains, Russia see **Figure 1**) [22, 23]: it is 70 km long, up to 2 km wide, and as much as 150 m thick. The formation of flows of such a length is possible during the outpouring of liquid basaltic lavas and at significant flow rates. It is known that, for example, in the recent eruptions on the Hawaiian islands and in the east African zift Zone (Congo) the lava flows moved at a rate of 40 and up to 100 km per hour [2].

Under conditions of a heavily dissected topography, lava flows tend to fill river valleys. In case the land surface is relatively flattened, the lava flow may be inconsistent with valley direction, as we can see with Undara (Queensland, Australia) and Thjorsa (Iceland) basalt flows [19, 21]. Lavas may flow across flat-bottomed shallow linear hollows, cover low watersheds, and form dams of a kind; as a result, the river appears completely dammed or its flow is forced aside. A phenomenon of that type has been described in the Colorado river valley (Mendoza province, Argentina), where El Corcovo lava flow blocked the river ~840 ka BP; later the stream formed a new incision about 1 km south of its former position [24].

As to morphologically distinct valleys, they may be completely or partially filled with lava, with both mainstream and its tributaries dammed. Because of the main river displacement and tributary valleys partly flooded, dammed lakes develop as we can see in the Bolshoy Anyuy river valley in Chukotka (the Northeast of Russia) [18]. They may be considerably deep (up to tens of meters), depending on the initial topographic dissection and the lava thickness. For example, dammed lakes 50–70 m deep existed in the lower parts of the Maly Yenisey tributaries (Eastern Sayan mountains, Russia—see **Figure 1**). Sedimentary sequences, including lava series, studied in the Maly Yenisey valley, provided evidence of dammed lakes having been common enough in the main valley and its tributaries; the subaqueous outflow of lava resulted in pillow lava and hyaloclastite formation [16].

The lakes of the Jom-Bolok drainage basin (the largest of them—Khara-Nur—is approximately 9 km2 in area) are drained under the lava at present (**Figure 4**) [25, 26]. In case such drainage is impossible, or it is less than the volume of water entering the lake, there may be several scenarios of further events. For example, the dammed lake may overflow into an adjacent valley, or even into another drainage basin [27].

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

#### **Figure 4.**

*Jom-Bolok lava flow (light gray), volcanoes (red), and dammed lakes (blue): 1 - existing lake Khara-Nur, 2 - drained paleolake Zun-Ukhergei. White dotted line—Watershed between the Oka and the Bolshoy Yenisey rivers basins (Eastern Sayan mountains, Russia—See Figure 1, No 9).*

#### **Figure 5.**

*Destroyed lava dams: A - the Hvita river, Iceland (2014); b - the Oka river, Eastern Sayan mountains, Russia (Figure 1, No 9). White arrow—The place of destroyed dam (2019, UVA photo courtesy by V. Pellinen).*

A similar situation periodically occurs in the upper reaches of the Jom-Bolok river, where the Khara-Nur Lake flows into the neighboring Bolshoy Yenisey river basin during periods of increased moisture. Examples of such river network restructuring are

described also in Ref. [28] for the Sikhote-Alin' and the East Manchurian mountains (Russia-see **Figure 1**).

The lava dam may be also eroded completely or partly by the stream [27, 29, 30]. The remains of lava dams and traces of drained reservoirs have been preserved in the valleys of the river Oka, Dzhida, and many others. Jom-Bolok lava flow dammed Oka river with Zun-Ukhergei paleo-lake formation (**Figure 5**). In any case, an active erosion (downcutting) would initiate a mud- or debris flow descent in due course in the valley.

Explosive eruptions are more diversified in their consequences. Practically each of them is accompanied by an ejection of considerable volumes of *pyroclasts*. The ejecta volume at colossal explosive eruptions (VEI-6) exceeds 10 km3 and may be more than 1000 km3 during mega-colossal ones (VEI-8); the latter are relatively rare (about once in 50 thousand years). They usually result in development of ignimbrite mantles covering the pre-eruption surface and forming plains over an area of hundreds and thousands of square kilometers. More common are relatively small eruptions, though they also can produce practically instantaneous changes in local topography. During the Shtyubel cone (Ksudach caldera, Kamchatka, Russia) eruption in 1907, for example, the volume of ejected pyroclastics is estimated at 1.5 to 2 km3 ; the tephra thickness varied from 0.5 to 3 m (**Figure 6a**) both in the immediate vicinity of the eruption center in the Ksudach caldera and at a distance of a few tens of kilometers from it (in the direction the wind was blowing during the eruption) [31].

In the late 1950s, Bezymyanny volcano (Kamchatka, Russia) ejected as much as 3 km3 of tephra; the deposits formed a cover up to 40 m thick over an area of 70 km2 and as thick as 40 cm over almost 500 km2 [32, 33]. In explosions, blocks weighing as much as a few tons may be ejected as far as up to 300 m from the vent of ejection, those weighing a few kilograms—over a distance of 3–6 km [2], and the smaller-size ones may be thrown as far as 20 km [34]. The ash layer more than 30–40 cm thick would cause drying up or loss of vegetation [35], that is, in turn, has an effect on the erosion and slope processes [36, 37]. Ashfalls introduce noticeable changes into the local topography, reducing slope steepness and changing soil characteristics. The pyroclastic layers deposited over river valleys and watersheds may result in essential changes in the valley network pattern, as the new erosional landforms would develop in accordance with the new relief (**Figure 6b**) and may disagree with former valleys.

The *pyroclasts* ejected during an eruption are noted for high porosity and, consequently, for lightness, so that the material may be easily transported by wind and water and *concentrates gradually* in topographic lows (primarily *in river valleys*). Abundant rainfall or snow melting bring about the descent of mudflows (with solid ingredient proportion of more than 60%) or hyper-concentrated flows, with proportion of solid ingredient between 20 and 60%, which gradually transport pyroclastic material downstream (**Figure 6c**). That is best illustrated by a concrete example of Bezymyanny volcano (Kamchatka, Russia) eruption on March 30, 1956: the ejected pyroclastic induced an active snow melt that resulted in mudflows up to 75–85 km long formed in the Sukhaya Khapitsa valley on slope of the Klyuchevskoy volcano (Kamchatka, Russia) [38].

Lahar deposits are usually accumulated at the base of the volcano slopes; the length of the flows may be considerable, up to 185 km (Kelud volcano, Indonesia, 1919) and even as great as 300 km (Cotopaxi volcano, Ecuador, 1877). Traces of lahars have been recorded on most of the active volcanoes of the world having typically the explosive type of eruptions (**Figure 7a**): to take a few examples, there are 22 events of that kind recorded on Cotopaxi slopes in sixteenth-nineteenth centuries [39]; 20 glacial-volcanic *Catastrophic Processes in River Valleys of Volcanic Regions: Geomorphologist's Point of View DOI: http://dx.doi.org/10.5772/intechopen.108141*

#### **Figure 6.**

*Water redeposition of pyroclastic material: A - slopes with gullies in pyroclastic cover (Ksudach caldera, Kamchatka, Russia, 2016—See Figure 1, No 12).); b - St. Helens volcano slopes with newly formed valleys in pyroclastic flow deposits (USA, 2018—See Figure 1, No 14); and c - modern lahars deposits in the Lagernyi creek valley (Ksudach caldera, Kamchatka, Russia, 2016).*

mudflows are known to occur on the Klyuchevskoy slopes (Kamchatka) in 1737 to 2008 time interval; there are 11 stages of large mudflows composed of melted snow and volcanic materials, which descend by the valleys on Shiveluch volcano (Kamchatka, Russia) southern slopes (the Kabeku, Bekesh, Baydarnaya, Kamenskaya, and other rivers) from 1854 to 2009 (**Figure 7b**) [11]. An eruption of the small Chaiten volcano (Chile) in 2008–2009 was responsible for three lahars; one of them (May 2009) inflicted damage on the city of Chaiten (**Figure 3**). Observations performed in valleys around volcanoes [2] proved that the valley bottom may be hazardous for a considerable length of time (several decades) after the eruption because of a lot of the unconsolidated sediments within and the expected subsequent lahar events in the valleys.

Quite often explosions occur not only with the ejection of pyroclasts but also with partial demolition of the volcanic cone. Even in the case of a small-size volcano, when its top is blown off, large blocks are scattered, and adjoining valleys may be dammed with coarse material. That often results in a dammed lake formation or in rising of the preexisting lake level. Such a case was recorded, for example, in 1907 in the uppermost reaches of the Teplaya river (Ksudach caldera, Kamchatka, Russia). Later, when the dam is broken (**Figure 8**), a mudflow descent occurs inevitably, which is confirmed by the presence of a large fan at the river mouth. Intracaldera lake breakout floods have been identified in the Taupo volcanic zone (New Zealand) also [40].

One-sided destruction of the volcanic cone during eruption—the so-called directed blast (or more neutral term - sector collapse)—is often accompanied by a debris avalanche development (see **Table 1**). As a result, non-sorted debris is deposited on the part of slope the explosion had been directed at. As noted by Ref. [34], the rock fragments may be thrown off over a distance of 29–30 km. A large debris avalanche goes as far as 85 km from the cone failure and covers an area of 100 to 1000 km2 [2]. The resurgent material may either infill river valleys completely over a considerable length or build up dams there. The case of a valley infilling was recorded during the Bezymyanny volcano eruption (Kamchatka, Russia) in 1956 when valleys on the eastern slope of the mountain were filled with debris and ejecta over a length of a few kilometers [32]. When streams resumed their activity, a series of copious mudflows developed and brought the material onto the right side of the Kamchatka river valley [38]. Debris avalanches in the Chakachatna river valley (Alaska, USA) resulted in the formation of long dams and dammed lakes with depth up to 150 m [14]. Not-so-huge lakes were formed in valleys around St. Helens volcano after 1980 eruption (**Figure 9a**). The avalanches descent during the Holocene Shiveluch volcano eruptions (Kamchatka, Russia) brought about a radical restructuring of the valley network: the Kabeku river captured a parallel water stream blocked with large blocks of the avalanche (**Figure 9b**) [41].

#### **Figure 7.**

*Lahars: A - on the Fuego volcano slope (Guatemala, 2013—See Figure 1, No 34); b - in the Kabeku river valley (Shiveluch volcano foot, Kamchatka, Russia, 2013—See Figure 1, No 13).*

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

#### **Figure 8.**

*A fragment of a destroyed debris dam (white arrow) at the source of the Teplaya river cause a rise in the level of Ksudach caldera's lakes by 15 m (white dotted line) after the eruption of 1907. The black dotted line shows the top of the Shtyubel cone destroyed by the 1907 explosion (Kamchatka, Russia, 2016—See Figure 1, No 12).*

#### **Figure 9.**

*Debris avalanches impact: A - dammed Coldwater lake in the Coldwater Creek valley with hummocky relief remnants (white arrow) as islands (St. Helens volcano, USA, 2018—See Figure 1, No 14); b - a debris avalanche deposits with soil-pyroclastic cover in Kabeku river valley (Shiveluch volcano, Kamchatka, Russia, 2013—See Figure 1, No 13).*

In explosive eruptions, a heavier part of the eruptive column may form pyroclastic flows—a mixture of burning hot (often above 600–700°C) blocks, ash, and volcanic gases. They descend from the eruptive vent downslope at a rate of 100 m/s or more [2] mostly following valleys of streams dissecting the slopes. There are known occasions when pyroclastic flows rose upstream valleys passing over the mountain ranges enclosing caldera; such a case was reconstructed [42] to have occurred during the latest caldera-forming eruption of Ksudach volcano (Kamchatka, Russia) 1725 yr. BP (**Figure 10a**). As a result, the valleys turn out to be filled with pyroclasts; later the loose pyroclastic mantle was eroded, and mudflows arose. The pyroclastic

flows affect great areas, up to ten and hundreds of square kilometers. After Shiveluch volcano eruption (Kamchatka, Russia) in 1964, the affected area was 45.5 km<sup>2</sup> [43]. In the succeeding years, the flows repeatedly descended by the stream valleys of the southern slope, and their length varied between 8 and 28 km [44]. Pyroclastic flows are known, however, to be as long as 100 km [1, 34].

On entering a valley, a pyroclastic flow covers its floor completely and, in common with lava flows, forms a convex transversal profile. On the Shiveluch volcano, the pyroclastic flow deposits (**Figure 10b**) vary between 2 and 5 m and 40 and 50 m [43–45]. The deposits may be loose or welded (as pumice and ignimbrites). Accordingly, the former is more easily destroyed by erosion and mudflow formation. Pumice and ignimbrite flow during *caldera collapse* often form plains over pre-existing landforms. However, they are seldom marked by considerable durability and their surface is often dissected by erosion to a stage of badland (**Figure 10c**).

In common with lava flows, the pyroclastic ones may block the tributaries at their entering the main valley and form dammed lakes (**Figure 11a**). The pyroclastic dams, however, are not very strong and may be broken by erosion and mudflows within a few years after the eruption. Such was the case of the southern slope of Shiveluch volcano (Kamchatka, Russia) [45]. As the streams are usually overloaded with loose pyroclasts, the deposition rate in the lakes is rather high; to take but one example, a series of horizontally stratified sands more than 6–7 m thick accumulated in the dammed lake at the Sukhoy Bekesh river per 3 years (**Figure 11b**). The lakes dammed by ignimbrites are long-lived and large, and their depth may be as great as 100 m like in Tadamy river-dammed lake (Japan). But this ignimbrite dam also was destroyed with debris (mud) flow formation along a river valley 150 km long [46]. Similar situations with ignimbrite dams were also reconstructed by [40] for Tarawera lake in Taupo volcanic zone (New Zealand).

During the observation period of the Shiveluch volcano activities (1964–2013), the lahar descent was always preceded by the pyroclastic flow eruption. This fact led I.B. Seinova and her colleagues [47] to the conclusion that the pyroclastic flows is a trigger mechanism in the lahar initiation. In 2009, the eruption of Sarychev peak volcano (Matua Isl., the Kuriles, Russia—see **Figure 1**, No 36) produced eight pyroclastic flows, which subsequently gave rise to seven mudflows (lahars) [48]. A regular lahar formation has been recorded after Merapi volcano (Indonesia) eruptions usually accompanied by pyroclastic flows. Ten out of 18 largest streams originated on the volcano became repeatedly the ways of lahar descent [5]. The studies of the pyroclastic flow deposits on St. Helens volcano showed that the pyroclastic flows (or pyroclastic density currents–PDC) exert a noticeable erosive effect on the substrate [49], particularly in case they move downward by linear hollows on steep volcanic slopes. Avulsions, riverbank erosion, and riverbed downcutting were presented as lahars geomorphic impacts at Merapi volcano river valleys after the 2010 explosion [5, 50].

After a large eruption accompanied by pyroclastic ejection, the solid runoff of rivers in volcanic regions may be several orders of magnitude greater than before the eruption, that is distinctly seen in the graphs constructed for Kamchatka and Tolbachik rivers (Kamchatka, Russia) [51]. According to Ref. [2], after St. Helens eruption in 1980, the annual solid runoff of rivers in the vicinities increased by a factor of 500, and even 20 years after the event the annual suspended load increased ~100 times as compared with the value before the eruption. The streams flowing from volcanoes are usually overloaded with rock debris varying in size, and great volumes of pyroclasts (including that redeposited by lahars) are to be transported by the rivers [52].

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

#### **Figure 10.**

*Pyroclastic flow (PF) deposits: A - traces PFs (black dotted lines) expand upstream (upslope) passing over the mountain ranges enclosing Ksudach caldera (2016); b - burnt birch trunks in a pyroclastic flow in Kabeku river valley, Shiveluch volcano (2013); and c - dissected Holocene pyroclastic plain near Kuril lake caldera (2016); all photos—Kamchatka, Russia (see Figure 1, No 12, 13, 20).*

**Figure 11.**

*Dammed lake between two pyroclastic flows (a - the Kabeku river valley) and destroyed dammed lake deposits (b - the Bekesh river valley), Shiveluch volcano slopes, Kamchatka, Russia (2013—See Figure 1, No 13).*

Observations in the valleys of the Kabeku and Bekesh rivers (the Shiveluch volcano slopes, Kamchatka, Russia) allow us to conclude that the frequent descent of lahars causes many changes not only in the nature of the runoff but also in the morphology of the valleys in the areas of their deposition.

In case the eruption occurs in a lake (within caldera or in a dammed water body in a valley), or in its immediate vicinity, no matter if it is underwater or above, it results in the water expulsion from the lake and a mudflow descent from the slopes or along the valley. According to Refs. [6, 7, 9, 12, 40], the generation of eruption-triggered lahars by the ejection of water from lakes is widespread in New Zealand and in other regions of the world. Since 1861 a lot of lahars have been generated from the Crater lake on Ruapehu volcano. The evidence of such phenomena is traceable in the Teplaya river valley (Ksudach caldera, Kamchatka, Russia) and in valleys of other rivers flowing out of volcanic lakes [53]. A subaqueous explosive eruption was observed in 1996 in Karymsky lake (the Akademia Nauk caldera, Kamchatka) with a series of tsunami to 15 m high and breakthrough floods from lake along Karymskaya river valley [54]. Then, at the source of the river, a dam of pyroclastic material arose, which after a few months was broken through with the descent of the lahar.

In the regions of the continental ice sheet, the eruption taking place under a thick ice cover may create giant outburst floods of meltwater known under the name of *jökulhlaup*. At present, they are known to occur in Iceland [55, 56]; during the Quaternary cold intervals, they seem to have happened in Kamchatka [57], as well as on the east Tuvinian lava upland (Eastern Sayan mountains, Russia—see **Figure 1**) [58] and in other volcanic regions of midlatitudes. When an eruption takes place under glacier, the meltwater forms a subglacial water body under considerable pressure; subsequently, the ice may subside and an outburst flood occurs accompanied by the abrupt release of great volumes of water, ice fragments, and stone debris being transported over a large distance, particularly along river valleys. Enormous volumes of water involved in the process account for the great scale of the phenomenon. For example, the length of the flows, their rate, and the transported material volume exceed the characteristics of lahars by orders of the value. For example, the Katla volcano eruption under Myrdalsjökull glacier in 1918 induced a jökulhlaup of 8 km3 volume and a flooded area of 600–800 km2 . Those flows maybe 20 to 70 m deep and 8–9 km wide. No river channel can hold a great volume of water. Quite often, the passage of the flood causes changes in topography and a large-scale restructuring of river network. That
