Deltas: Landforms, Ecosystems and Human Activities Proceedings of HP1, IAHS-IAPSO-IASPEI Assembly, Gothenburg, Sweden, July 2013 (IAHS Publ. 358, 2013).

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Changes in hydrological regime and morphology of river deltas in the Russian Arctic DMITRY MAGRITSKY, VADIM MIKHAILOV, VLADISLAV KOROTAEV & DMITRY BABICH

Moscow State University, Faculty of Geography, PO Box 119991, Leninskie gory, GSP-1, Moscow, Russian Federation magdima@yandex.ru Abstract This paper provides information on the current regime and morphological processes in the river deltas of the Northern Dvina, Pechora, Ob, Pur, Taz, Yenisey, Olenek, Lena, Yana, Indigirka and Kolyma, and their possible changes in the 21st century. This article contains data on morphological type, structure and size of the major deltas, about the current state and changes of the main river (water and sediment river runoff) and marine factors of delta formation processes. Key words river delta; water and sediment runoff; hydrological regime; morphological processes INTRODUCTION

The Russian Arctic occupies a large area (3.35 million km2 of land) and extends from west to east for more than 7000 km. About 10% of the continental coast of the Russian Arctic is covered by river deltas (Geoecological state of the Arctic coast of Russia and safety of environmental management, 2007). Despite their small area compared with the size of the entire region, these arctic deltas are extremely important to the Russian Arctic. This is especially true for the few river deltas. Deltas are a place of concentration of population, transport and industry; are the link between sea and river cargo transportation; are characterized by an abundance of water, biological and other natural resources; and have unique ecosystems that play important roles as a natural filter, detaining river sediment contaminants. At the same time, the vulnerability of ecosystems and environmental management instability in the Arctic deltas is very high. First, the river deltas are located in a zone with severe climatic conditions that result in very slow restoration of any disturbed environments. Second, the deltas are located at the junction of land and sea, so the state of, and processes in them, largely depend on processes in the river basins and seas. In the last decades, there were hydro-climatic changes, hydraulic engineering and water-related activities in the river basins, changes of the regime of the seas, and natural and anthropogenic changes within the deltas. These changes, firstly intensify some processes in deltas and limit others, and secondly, require a comprehensive and detailed study to better understand present and future change. GENERAL CHARACTERISTIC OF THE ARCTIC DELTAS OF RUSSIA

Each Arctic delta is individual in morphological type, location, size, structure, environment and regime. The largest deltas on the Russian Arctic coast are located at the mouths of the Severnaya Dvina (Sev. Dvina), Pechora, Ob, Pur, Taz, Yenisey, Olenek, Lena, Yana, Indigirka and Kolyma rivers (Fig. 1). The Lena River delta is the largest river delta in Russia (32 000 km2) and the fifth largest in the world. The delta length (along the main branch) is 175 km, while the length of the delta coastline (DCL) is 560 km. The Yana River delta (area is 6600 km2, the delta length is 140 km and the length of the DCL is 150 km), Indigirka River delta (5000 km2, 130 km, 170 km), Yenisey River delta (4500 km2, 196 km, 45 km), Pechora River delta (3250 km2, 120 km, 60 km), Ob River delta (3250 km2, 144 km, 136 km) and Kolyma River delta (3250 km2, 120 km, 90 km) are also of considerable size (Mikhailov, 1997). The smallest deltas are the Olenek River delta (550 km2), and the Severnaya Dvina, Pur and Taz rivers have deltas, areas of which are 900, 630 and 830 km2, respectively. The Arctic river deltas can be classified as: (1) deltas in lagoons and bay-head (estuarine) deltas (Pechora, Ob, Pur, Taz and Yenisey), (2) protruding (advanced) deltas in an open nearshore

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zone with expansion of mouth bars and separate delta lobes for the general sea coastline (Sev. Dvina, Yana, Indigirka and Kolyma), and (3) classic protruding deltas in an open nearshore zone (Lena and Olenek), the territory of which is mainly the indigenous sea coast. All deltas are in a multi-branch phase of development. In the Lena River the number of watercourses and nodes of the delta channel network are equal to 170 and 51. The mouth nearshore zone (MNZ) of Sev. Dvina, Olenek, Lena, Yana, Indigirka and Kolyma is open and steep, while other rivers have semi-enclosed (lagoon, bay, inlet) and a shallow MNZ.

Fig. 1 Russian Arctic. The delta of: 1, Severnaya Dvina; 2, Pechora; 3, Ob; 4, Pur; 5, Taz; 6, Yenisey; 7, Olenek; 8, Lena; 9, Yana; 10, Indigirka; 11, Kolyma rivers.

The deltas are located on the coast of the Barents, White, Kara, Laptev and East-Siberian seas, to the north of the Arctic Circle (except for the delta of Sev. Dvina), in a temperate (Sev. Dvina, Ob), subarctic (Pechora, Pur and Taz) and Arctic climatic zones. The geographical position of the deltas largely determines the features of their environment and socio-economic development. Almost all deltas are characterized by cold and severe climate, the presence of permafrost and permafrost processes, good availability of water resources, multi-element and dense hydrographic networks, a long period of ice cover, the weak development of the territory, uniqueness and vulnerability of mouth ecosystems. An exception is the Sev. Dvina River delta, which is located outside the zone of permafrost and the main part of the territory and is occupied by the Arkhangelsk and Severodvinsk cities with a population of about 550 000, and developed industry. Landscapes of considered Arctic deltas consist of lowland terrain (with an average elevation of 0 to 5–15 m above sea level), with permafrost and alluvial forms of relief, of tundra plant communities with few trees along rivers and delta branches, of large number of watercourses, thermokarst and flood plain lakes, wetlands, with alluvial, tundra and marsh soils affected by permafrost processes. The only exception is the Sev. Dvina delta which is significantly urbanized and located in the north-taiga plant subzone. HYDROLOGICAL FACTORS OF DELTA FORMATION PROCESSES The structure and regime of the Arctic deltas depend on the hydrological regime of the rivers and the seas. The major river factors are water flow and sediment runoff of rivers and their changes; the main marine factors are wind waves, ocean currents, sea-level changes. Other important hydrological factors are the temperature of river and sea waters; ice extent in the coastal zone of the Arctic seas, seasonal and long-term changes of these characteristics.

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The water runoff and its changes

Average annual runoff of the Sev. Dvina, Pechora, Ob, Pur, Taz, Yenisey, Olenek, Lena, Yana, Indigirka and Kolyma is 108, 132, 408, 32.9, 45.8, 633, 40.7, 543, 35.9, 54.1 and 124 km3, respectively (see Table 1). In sum, it is 75% of inflow of the total river waters from the Russian part of the Arctic Ocean basin. Table 1 Water runoff (estimated up to 2005) and sediment runoff (estimated up to 2007) of the large Arctic rivers of the Russia. The rivers Lowest hydrometrical station Upper boundary of the delta Marine boundary of the

delta Area of river basin (1000 km2)

Water runoff (km3/year)

Suspended sediment runoff (1000 t/year)

Mean water turbidity (g/m3)

Suspended sediment runoff (10 × 106 t/year)

Bottom sediment runoff (10 × 106 t/year)

Area of river basin 1000 km2)

water runoff (km3/year)

Sev. Dvina 348 105 3270 30.8 3.33 0.65 357 108 Pechora 248 110 5590 51.4 6.43 2.28 322 132 Ob 2953 398 15900 40 16 2.89 2990 408 Pur 95.1 28.4 707 25.1 0.77 0.41 112 32.9 Taz 100 33.5 (524) (15.6) 0.73 0.49 150 45.8 Yenisey 2440 587 120001–41002 22.01–6.82 12.41–4.52 2.77 2580 633 Olenek 198 37.2 1160 33.6 1.31 1.12 219 40.7 Lena 2430 533 21200 40 21.4 5.40 2490 543 Yana 224 34.4 4480 130 4.49 1.46 238 35.9 Indigirka 305 50.5 11700 230 11.8 3.40 360 54.1 Kolyma 526 104 9940 98.2 11.7 4.20 647 124 1before the period of regulated regime, 2during the period of regulated regime.

Fig. 2 Time series of mean water runoff (1) and suspended sediment runoff (2) of the large Arctic rivers.

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The magnitude of Arctic river runoff is changing in response to the intra-annual and long-term changes (in river basins) of the main factors of runoff formation. The main feature of long-term fluctuations of water flow of most of the Arctic rivers is an increase (Fig. 2). The main reason for this tendency is regional changes in climate conditions since the late 1970s–early 1980s, and in particular 1988–1989 (Gruza & Ran’kova, 2003; Geoecological state of the Arctic coast of Russia and safety of environmental management, 2007; Assessment Report on Climate Change and its Consequences on the Territory of the Russian Federation, 2008a; Water resources of Russia and their use, 2008). The increase in annual runoff was the most significant at the Yenisey, Khatanga, Anabar and Olenek, and is instrumentally recorded since the 1980s. An increase in runoff of rivers of the western part of the Kara Sea basin and the Kolyma River is observed from the 1990s. Intra-annual structure of changes of annual water runoff differs at the different rivers. At the mouths of the Sev. Dvina, Pechora, Lena and Olenek an increase in annual water runoff was caused by increase of water runoff in almost all seasons of the year. Water runoff of the Ob, Pur, Taz, Yana and Indigirka in spring–summer flood time increased in 1976–2006 by 1.5–3% (in comparison with 1936–1975). Water runoff of the summer–autumn period increased at the Yana River and Indigirka River (20–25%) and to a lesser extent at the Kolyma River (2.5%). The reason for this was not only precipitation, but also the degradation in the basin of these rivers of ice crust foundations, glaciers, snowfields and underground ices (Assessment Report on climate change and its consequences on the territory of the Russian Federation, 2008b; Water resources of Russia and their use, 2008; Alekseev et al., 2012). Almost all the rivers (except for Yana and Indigirka) had positive dynamics of winter flow. A significant increase in winter flow was observed at the Sev. Dvina, Pechora and Ob (6.5–14.7%) and especially at the Yenisey, Lena and Kolyma (68, 37 and 136%, respectively). Increase of winter water flow of the Ob, Yenisey, Lena and Kolyma is connected with climatic factors and artificial regulation of runoff of these rivers and their major tributaries (Magritsky, 2001, 2008). Operation of the large reservoirs has been an important factor in the reduction of the water flow of the Yenisey River and Kolyma River in spring, summer and autumn. At the lowest hydrometrical station of the Yenisey River (gauge Igarka) water discharges in November–March and April in 1970–2001 amounted to 158 and 213% of their values in 1936–1961, water discharges in May–July amounted to 92%, water discharges in August–October amounted to 86%. At the Srednekolymsk gauge in the lower reaches of the Kolyma River water discharges in November–April in 1992–2001 amounted to 334% of their values in 1948–1980, 92% in May–June, and 92% in July–October. Construction of reservoirs, measures for intake and diversion of runoff slightly affected the annual runoff of Arctic rivers. Changes in water resources and water regime of the Arctic rivers in the 21st century will continue (Arnell, 1999; Mokhov et al., 2003; Geoecological state of the Arctic coast of Russia and Safety of Environmental Management, 2007; Bates et al., 2008; Magritsky, 2008; Meleshko et al., 2008; Water resources of Russia and their use, 2008). To a lesser extent they will be caused by an increase of the scale of economic activities in the river basins. The exception will be the Ob River, within the basin of which large-scale projects of diversion of water runoff are being implemented. The main changes in runoff Arctic rivers in the 21st century will be connected with the change of regional climatic characteristics. The majority of researchers agree that water runoff of the Arctic rivers most likely will increase (Assessment Report on climate change and its consequences on the territory of the Russian Federation, 2008b; Water resources of Russia and their use, 2008). By the middle of the 21st century, the relative increase of the water resources of the Sev. Dvina, Pechora, Ob, Yenisey and Lena could be from 4 to 14% and more. A particularly significant increase is expected in runoff during the winter season, first of all at the rivers of basins of the White and Barents seas. Sediment runoff of the rivers and its long-term changes

River sediment runoff is one of the most important factors of delta formation. The waters of the Arctic rivers in their lower reaches and mouths have low turbidity (<30–50 g/m3). The largest

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values of suspended sediment concentrations are Yana (130 g/m3), Indigirka (230 g/m3) and Kolyma (98 g/m3) (see Table 1). The low turbidity of river waters is caused by long cold season, wide spread of permafrost, resistant to erosion of rocks, the relatively low range of elevation, abundance of wetlands and lakes, large wooded territory and other factors that limit the intensity of erosion processes in the river basins (Geoecological state of the Arctic coast of Russia and safety of environmental management, 2007; Magritsky, 2010). Most of the suspended sediment transits to the Lena River delta (21.4 million t/year), with smaller amounts of sediment transit to deltas of the Ob (16), Yenisey (12.4), Indigirka (11.8) and Kolyma (11.7 million t/year) (see Table 1). About 99% of the total volume of suspended sediments, inflowing into the Arctic deltas, are sediments of the main river (in the delta head) and only 1% are sediments of the rivers that flow into the boundary delta branches. From 30 up to 50% of suspended sediment remains in Arctic deltas. Bottom sediment runoff of the Sev. Dvina, Pechora, Ob, Pur, Taz, the Yenisei, Olenek, Lena, Yana, Indigirka and Kolyma is approximately equal to 0.65, 2.28, 2.89, 0.41, 0.49, 2.77, 1.12, 5.4, 1.46, 3.4 and 4.2 million t/year (Geoecological state of the Arctic coast of Russia and safety of environmental management, 2007). The main part of bottom sediments (up to 80–90%) is deposited and redeposited in the delta watercourses. Only a small part of them reaches the delta coastline. Fluctuations of suspended sediment discharges usually reflect changes in water flow, but the intra-annual and inter-annual variability of sediment discharges is much higher than for water flow. Nevertheless, the increase of water runoff of Arctic rivers is accompanied by an increase in suspended sediment runoff (Fig. 2). This tendency is not specific to regulated rivers (Magritsky, 2010). For example, transport of suspended sediment into the deltas of the Kolyma River and the Yenisey River was reduced after the construction of the Kolyma reservoir and the Angara-Yenisey reservoirs by two and three times, respectively. In the future we should expect an increase in sediment runoff in the Arctic seas. It will be promoted by an increase in water runoff and maximum water discharges of rivers, and air and water temperature. According to the most approximate estimates, the annual suspended sediment runoff of the Sev. Dvina, Ob, Olenek, Lena, Yana and Indigirka may increase (in the case of increase of the average water runoff at 5 and 10%) by 8%/17%, 6.8%/13.7%, 2.7%/5.3%, 12%/25%, 12%/24% and 5.8%/11.5%, respectively. In the mouths of the regulated rivers, present-day and future regime of suspended sediments will depend on the operation of large hydraulic engineering constructions. The value of the sediment yield in the river mouths may impact mining activities. The water level in the Arctic seas

Changes of level in the Arctic seas are composed of long-term, seasonal and short-term fluctuations. As a result of global warming and its consequences for the hydrosphere and the cryosphere, the level of all four oceans, including the Arctic Ocean and the Arctic seas, is rising (IPCC, 2007). Mean annual level of the Arctic Ocean rose in 1946–1995 at a rate of 1.5 mm/year (Vorobyev et al., 2000). The majority of this rise has occurred since the 1980s, and coincides with the beginning of the warming in the region and an increase in water flow of the Arctic rivers. The values of a linear trend of changes in the level vary for different Arctic seas. This is explained by the reaction of the measured level, not only to change in sea water volume, but also on the vertical movements of the sea coast on which there are hydrological gauges. Therefore, it is the relative sea level rise (RSLR). For the Barents, Kara, Laptev, East Siberian and Chukchi seas RSLR was 0.2, 1.5, 2.1, 1.5 and 2.3 mm/year (1946–1995), respectively. In the future, growth of the World ocean level and of the Arctic Ocean and Arctic seas will continue. Tides at the nearshore zone of the Arctic rivers in most cases are symmetrical and semidiurnal (Mikhailov, 1997; The National Atlas of Russia, 2007). The value of the tidal level variation (in spring tide) is reduced from 1.3 m at the Sev. Dvina River mouth and 1 m at the Pechora River mouth to 0.7 m in the Ob-Taz bay, to 0.6 m in the coastal zone of the Olenek and Lena deltas, to 0.2–0.3 m on the Yana and Indigirka delta coastline and to 0.1 m at the Kolyma River mouth. The

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furthest tidal fluctuations penetrate into the Sev. Dvina River (135 km), Pechora River (190 km) and Yenisey River (445 km). The length of the tidal reaches of the Ob, Yana and Indigirka are 51, 30 and 24 km. The maximum value of water level rise near the coastline of the Arctic deltas during storm surges is in the range of 1.5–3.0 m (Mikhailov, 1997). The number of negative storm surges in the Arctic river mouths is much less than the number of positive storm surges. The maximum value of negative storm surges is smaller than the value of positive storm surges. For example, negative storm surges in the Sev. Dvina River mouth have a maximum value of 0.8–0.9 m, and the value of positive storm surges is 1.7–1.9 m. In the Pechora Bay, the maximum value of positive storm surges is equal to 2.4 m, and the value of negative storm surges is 1.2 m. The distance of penetration of storm surges level variations into the Arctic rivers has the greatest value at the Yenisey (870 km), i.e. more than at any other river in the world. At the Sev. Dvina, Pechora, Ob, Pur, Taz, Yana, Indigirka and Kolyma, the length of storm surges section is 135, 160, 350, 100, 180, 70, 200 and 282 km, respectively. Together with the storm surges and tides and in low water periods, seawaters penetrate the Arctic rivers. The distance of penetration of seawaters into the Sev. Dvina, Pechora, Ob, Pur, Taz and Yana is 45, 10, 0, 0, 0 and 60 km, respectively (Mikhailov, 1997). REGIME AND MORPHOLOGICAL PROCESSES IN THE ARCTIC DELTAS

Water regime and morphological processes in the Arctic deltas include intra-annual and inter-annual changes of water runoff and levels, the distribution and redistribution of water flow between delta watercourses, flooding of delta land by river and sea waters, the water balance of the deltas, channel processes and dynamics of the hydrographic network in general, and dynamics of the DCL. The scale and intensity of these phenomena and processes depend on the river and sea factors, their seasonal and inter-annual variability, location, structure and natural conditions of the delta. The regime and morphological processes in the delta may influence the local economic activity. Distribution of river runoff in the channel network of deltas

The distribution of water flow between delta branches and their systems is determined, first of all, by the ratio of hydraulic and morphometric characteristics of branch channels (Mikhailov, 1998). The percentage of runoff of the shorter branches, with bigger width and especially depth, with smaller roughness of its channel, is higher. The width and depth of the branches vary due to seasonal changes in water supply of the deltas. This leads to a redistribution of water flow between the branches during the year. For example, the increase of water discharges of the Lena River from 8500 to 85 000 m3/s leads to a decrease in percentage of runoff of the Trofimovsky branch from 71.4 to 53.2% (see Table 2). In contrast, the relative runoff of the Oleneksky and Tumatsky branches increases from 4.1% to 9% and from 2.2% to 9.7%, respectively. A decrease in percentage of branch runoff during the seasonal increase in river water flow (e.g. branches Trofimovsky in the Lena River delta, Mesin in the Pechora River delta, Taz in the Taz River delta, etc.) indicates the existence of tendencies of its activation. Such branches usually have bigger depths and are suitable for navigation. Tides (Sev. Dvina and Pechora deltas) and storm surges (all deltas) have an impact on runoff distribution between delta watercourses within its sea-side reaches. For example, wind rise of level near the Ob delta coastline leads to a reduction of percentage of runoff of Hamanelskaya Ob, and a level fall leads to its increase (Piskun, 2002). This regularity is well expressed during the low flow period. In contrast, for water discharges ≥15 000 m3/s this influence becomes insignificant. In the delta branches directly near the sea, this effect is maintained, even during maximum water discharges. The distribution of the suspended sediments between delta branches is proportional to the distribution of water flow. Some increase in the proportion of suspended sediment runoff, in comparison with the distribution of runoff water, can be observed in passive and dying delta

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Table 2 Data on the distribution of water runoff in the Arctic deltas of Russia in the second half of the 20th century and at the beginning of 21st century (in the conditions of absence of ice formations). The rivers The main delta branches Tendencies of

development and types of delta branches

Relative distribution (%) of water runoff between the delta branches Ratio of river water discharge to the long-term mean annual river water discharge 0.25 0.5 1.0 1.5 2.0 3.0 5.0

Sev. Dvina Delta head: Nikolsky Murmansky Korabelny Maimaksa Kuznechiha Rybolovka

–2%/10 years –1.1%/10 years ~0%/10 years +1%/10 years ~0%/10 years –

100 38.0 20.2 24.0 13.0 1.8 3.0

100 32.2 19.7 21.6 19.6 3.1 3.8

100 29.3 18.2 20.9 22.3 4.9 4.3

100 29.0 18.0 21.0 21.8 5.7 4.5

100 30.6 18.0 21.0 19.7 6.2 4.4

100 32.4 17.8 21.3 17.4 6.7 4.5

100 34.3 17.3 22.4 14.8 6.6 4.6

Pechora Delta head: Bolshaya Pechora Malaya Pechora Andeg node of bifurcation: Utcher-Shar Krestovy Shar Sredny Shar Mesin

active passive passive passive passive active

100 59.8 40.2 100 8.4 0 9.1 82.5

100 56.7 43.3 100 10.2 0 24.8 65.0

100 54.7 45.3 100 11.1 0.7 33.4 54.8

100 53.4 46.6 100 11.3 2.2 35.6 50.9

100 53.0 47.0 100 11.7 3.3 37.4 47.6

100 53.1 46.9 100 12.7 4.5 39.8 43.0

100 54.9 45.1 100 14.8 5.4 42.9 36.8

Ob Delta head: Hamanelskaya Ob Nadym Ob

+1.2%/10 years –1.2%/10 years

100 56.0 44.0

100 54.0 46.0

100 52.5 47.5

100 51.0 49.0

100 49.0 51.0

Yenisey Muksuninsky cape: OkhotskYenisey Maly Yenisey Bolshoi Yenisey

passive passive active

100 3.8 16 77.4

100 3.5 22.5 70.2

100 3.3 23.7 68.8

100 3.3 23.5 68.6

100 3.2 22.7 68.8

Lena Island Stolb: Oleneksky Tumatsky Trofimovsky Bykovsky

+0.09%/10 years +0.11%/10 years –0.1%/10 years ~0

100 4.1 2.2 71.4 22.3

100 5.3 4.2 66.4 24.2

100 6.0 5.3 63.6 25.1

100 6.6 6.2 61.5 25.8

100 7.5 7.6 58.2 26.7

100 9.0 9.7 53.2 28.0

Yana Delta head: Pravaya Glavnoe Ruslo Remaining branches (6)

active active dying

100 25.4 49.7 24.9

100 23.6 45.1 31.3

100 22.2 42.6 35.2

100 20.5 38.4 41.1

Indigirka Delta head: Russko-Ust’inskaya Sredniya Kolymskaya

passive passive passive

100 37.2 62.8 8.9

100 38.5 61.5 7.6

100 39.3 60.7 7.6

100 39.7 60.3 8.7

100 36.2 63.8 10.5

Kolyma Delta head: Kamennaya Poperechnaya II Selivanovskaya Pohodskaya+Chukochia

– 100 38.9 24.3 1.4 35.4

According to (Hydrology of the Severnaya Dvina River mouth area, 1965; Ivanov & Gilyarov, 1972; Polonsky, 1984; Polonsky & Kuzmina, 1986; Alabyan et al., 1991; Lower Yana: mouth and channel processes, 1998; Babich et al., 2001; Magritsky, 2001; Piskun, 2002; Rudykh, 2004; Geoecological state of the Arctic coast of Russia and safety of environmental management, 2007). branches. Such regularity was revealed at the Nadym Ob (in the Ob River delta), the Maly Yenisey (in the Yenisey River delta), the Oleneksky (in the Lena River delta), the Ularovsky (in the

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Indigirka River delta) and others. Bottom sediments are distributed more in branches with a deeper channel in their source. Long-term redistribution of water flow in the delta happens due to natural or anthropogenic changes of parameters of channels and branches, the value and regime of river runoff, and changes of the sea regime. For the Arctic deltas low intensity of redistribution of water flow (no more than 1–2%/10 years) is inherent (see Table 2). It is caused by the low turbidity of river waters and a slower rate of channel deformation in the Arctic deltas. The period of the existence of watercourses in the Arctic deltas is estimated as decades at minor watercourses and centuries at the large branches. This is confirmed by the results of analysis of multi-temporal cartographic materials. For example, in the Yenisey River delta just one stage of dying of the Okhotsk Yenisey branch lasts about 150 years (Alabyan et al., 1991). The main features of current morphological processes in the Arctic deltas

Morphological processes include channel processes, general dynamics of the hydrographic network of deltas, and changing the shape of the sea coast, the position and size of mouth bars. The intensity of the morphological processes in the Arctic deltas, in comparison with the southern deltas of Russia, is relatively small. It is limited by the small turbidity of river waters and amount of sediments, long periods of negative temperatures, and freezing-up. But there are several factors, which are not executed to others deltas and under favourable conditions (in the case of climate warming, an increase of the temperature of the river and marine waters, changes of ice regime) may intensify the morphological processes. These are underground ice, high ice content in rocks and deposits, and scarce vegetation cover. Also, an important distinctive feature of the Arctic deltas and the processes occurring in them is a frozen reformation of inter-branch areas of delta plain with creation of thermokarst depressions, polygon relief forms, and hydrolaccoliths. Another feature is the participation in reformation of banks and bed of delta branches of floating ice, which destroys the banks directly due to damage of vegetation cover. Without vegetation cover the rate of thermal erosion of the branch banks increases 2–3 times. Channel reformations have seasonal (reversible) and long-term (usually irreversible) character. The intensity of fluvial processes increases during the spring high water and ice break-up, during the summer-autumn floods, on the reaches of significant changes in water flow (in the nodes of furcation and confluence), on sites of prevalence of easily eroded rocks and deposits, and in sea-side parts of the delta (in the zone of influence of marine factors). Small tortuosity of the delta branches and the relative invariability of the distribution of water flow between watercourses for long-term periods indicate the low intensity of channel processes in the Arctic deltas. The maximum intensity of erosion and inwash of the branch banks in the Arctic deltas is equal to 2–10 m or more (Geoecological state of the Arctic coast of Russia and safety of environmental management, 2007; Hydrology of the Severnaya Dvina River mouth area, 1965; Kravtsova & Cherepanova, 2003; Babich et al., 2001; Estuarine-deltaic systems of Russia and China: the hydrological–morphological processes, geomorphology and evolution forecast, 2007, etc.). The average rate of erosion of the banks is 3–4 times less. Rates of horizontal displacement of the channels of the delta watercourses are insignificant. In the Sev. Dvina River delta the value of displacement of the three major branches in the east direction (as a result of lateral erosion) was 5–7 km over a period of about 5000 years. In the middle and upper part of the Pur River delta the rates of meandering are 7–8 m/year. In the Taz River delta in the last 100–150 years there has been stability of the plan position of the main channel forms; new delta branches were not formed. In the Yana River delta a lateral displacement of branch bends with an average rate of 3.3 m/year is combined with its longitudinal evolution (up to 6 m/year); a fast displacement of meander bars leads to activation or dying off of minor watercourses (for a period of 7–15 years) at the reaches of channel branching. Outlines and location of midstream sandbanks, spits, meander bars and islands change at a greater rate than the intensity of horizontal deformations of the channels. The rate of erosion of island heads and island tails in the Yenisey River delta was 5–10 and 25 m/year, in the Lena River

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delta (in the system of Bolshoi Trofimovsky) 30 and 30–40 m/year, and in the Indigirka River delta 10–20 and 8–10 m/year. In the Yana River delta the meander bars move with a rate up to 20–60 m/year. In the Indigirka River delta the rate of growth of meander bars is 5–20 m/year, and the rate of displacement of midstream sandbanks amounts to 25–30 m/year. In active delta watercourses, erosion and deepening of channels prevail. In the navigable Maimaksa branch (in the Sev. Dvina River delta) this process progresses at the rate of 1.8 cm/year (Hydrology of the Severnaya Dvina River mouth area, 1965). In passive and dying delta watercourses accumulation of sediments and rising of bottom levels prevail, for example, in the Korabelny branch (0.7–1.9 cm/year) and the Nikolsky branch (0.7–2.4 cm/year) in the Sev. Dvina River delta. The character of the vertical deformations changes with the change of water content of the year or long-term period. The protruding of the Arctic deltas on the MNZ is developing very slowly, despite a large Arctic river flow. The reasons are small sediment runoff and their accumulation in the branches and on the surface of the Arctic deltas, coastal subsidence, the strong impact on the dynamics of sediments and sea coast of tides and tidal currents, waves and storm surges. Land subsidence is inherent for the territory of the Sev. Dvina River delta (up to –4 mm/year), the Pechora River delta (–4 to –2 mm/year), the deltas of the Ob-Taz bay and the Yenisey River (–30 to –24 mm/year) (Borisov, 1973, 1976; Geoecological state of the Arctic coast of Russia and safety of environmental management, 2007; The National Atlas of Russia, 2007). The Lena River delta is relatively stable. The territory of the Kolyma River delta is rising at a rate of about 0 to +4 mm/year. The Yana River delta may experience rising. The small capacity of the semi-enclosed MNZ contributes to the relatively rapid advancement of the coastline of some deltas. Therefore, the rate of linear protruding of the Pur, the Taz and the Yenisey deltas in the Taz bay and the Yenisey gulf is the largest among the Arctic deltas and is equal to 10–20, 15–20 and 25 m/year, respectively (Kravtsova & Cherepanova, 2003; Estuarine-deltaic systems of Russia and China: the hydrological–morphological processes, geomorphology and evolution forecast, 2007) (Fig. 3). The anthropogenic reduction of suspended sediment runoff of Yenisey River had little impact on the character of the dynamics of its DCL. The following is known about the dynamics of the marine edge of the rest of the deltas. The coastline of the Sev. Dvina River delta intensively moved forward near the Pogany and the Pudozhemsky mouths (Hydrology of the Severnaya Dvina River mouth area, 1965). In the mouth of the Pechora River the cone of the Bolshoi Pechora most intensively advanced in the Pechora bay (Mikhailov, 1997). The DCL of the Lena River near the mouths of the Oleneksky and the Trofimovsky branches, according to A. Gorelkin, almost did not change its shape for the last 30 years. The changes of coastline are expressed in the forming of several small islands at the mouths of these branches with a sum area of about 0.08% of the total area of the Lena delta and at a distance of 0.5–1.3 km (on the western site) and 1.5–7.7 km (on the eastern site) from the mainland coastline. On the site of Lena DCL, relating to the Karghinskaya marine terrace and containing ice, in contrast there is a retrogradation of the coastline with a rate of 0.7–2.5 m/year (1971–2003) (Grigoriev et al., 2006). Such a retrogradation is common for all delta shores that are located far from the mouths of the main delta branches, composed of permafrost and underground ices, represented by high and steep terraces. Climate and water warming increases the rate of the retrogradation of shores from 2 to 5 times. Intensive protruding of the Yana DCL is observed near the mouths of delta main branches. Near the mouth of the Pravy branch there is a growth of tails of three large Islands (total area of accumulation was 4.1 km2 in 1980–2007). In the mouth of the Glavnoe Ruslo branch the rate of protruding of land was 90 m/year, and the rate of increase of area was 27.6 km2. However, there are sites of erosion of the sea coast. Dynamics of mouth bars is expressed in the change of their size and position. Usually for a long-term period the elevation of bar crest increases, and the depths over the bar decrease. Average rates of increase of altitude marks of the bar crests in the Arctic deltas are usually insignificant and amount to about 0.01–0.1 m/year. In contrast, the horizontal displacement of the separate elements of mouth bars reaches significant values. In particular, a large rate is fixed in the case of protruding

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the spits at the mouth in shallow MNZ. At the Yenisey MNZ the surface parts of the spits increase in length at 10–30 m/year, and the underwater parts of the spits increase by 60–120 m per year. In the mouth of the Glavnoe Ruslo branch in the years with numerous and high floods, the layer of sediment accumulation on the bar is 1.0–1.2 m in trenches and 0.4–1.7 m at the bar crest. In such years the marine edge of a bar can advance in the MNZ by 300–400 m. In the absence of summer floods or insignificant runoff during the floods the bar trenches are noticeably deepened, and the width of the bar crest decreases. The long-term rate of deformation of mouth bars is less than its seasonal values. For example, the average growth rate of the bar in the mouth of the Glavnoe Ruslo branch was approximately 30–40 m/year (in 1981–1999). In the mouth of the Sredniya branch (in the Indigirka River delta) the bar grew from 1957 to 1990 at a rate of 30 m/year. Dredging works ambiguously influence the dynamics and morphological structure of bars in the mouths of the navigable branches.

Fig. 3 Dynamics of the hydrographic network and the marine edge of the Pur River delta from 1955 to 1999 (Kravtsova & Cherepanova, 2003). 1, delta islands; 2, branches, large streams, lakes, and bays; 3, small streams, and watercourses on the delta islands; 4, districts of land on the site of the water (the accumulation of sediment, the disappearance of lakes or decrease in their area); 5, disappeared watercourses; 6, areas of the appearance of water on the site of land (erosion of the banks, the appearance of a lake or an increase in their area).

Expected changes of the regime and the structure of the Arctic deltas in the future

The main changes of the regime and the structure of the Arctic deltas will be connected with the changes of Arctic river runoff, the thermal and ice regimes of the rivers and the coastal zone, with warming in the region and degradation of permafrost, the rise of the average level in the Arctic

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seas, and the increase in scale of economic activity in the deltas. These changes will have different socio-economic and environmental consequences: (i) The consequences of increasing river water runoff will be variable for the Russian Arctic

deltas. Firstly, redistribution of water flow between delta branches will take place. The relative value of runoff will increase at those branches in which the percentage of runoff is increased from low water period to flood period. Secondly, increasing water discharges will lead to an increase in longitudinal slopes and flow velocity in the branches, to intensification of fluvial processes. Thirdly, the increasing water runoff of the rivers will weaken the impacts induced by storm surges and tide level rise, reducing the frequency and distance of penetration into the branches of marine waters.

(ii) Increasing of maximum water discharges will increase the frequency and dimensions of the flooding of the delta flood plain.

(iii) Increasing water temperature and discharges, flow velocity in the river and the delta branches will intensify with bottom erosion, and thermal and mechanical erosion of the banks. This is already happening in the Yana River delta, where the total area of the erosion (11.34 km2) of the banks of the Pravy and the Glavnoe Ruslo branches in rainy and relatively warm years (1980–2007) exceeded the area of accumulation (3.63 km2). In the Lena River delta, for the same period, the erosion was double the accumulation.

(iv) Changes in water and temperature regime of the rivers, and climate warming will influence the ice regime of delta watercourses and MNZ. The weakening of ice regime will positively affect the duration of the navigation period in deltas. However, there will be an increase in the duration of the period with active fluvial processes and processes of reformation of the sea coast. Changes in ice regime can influence a situation, by formation of ice jams and ice jam inundations in the deltas, both positively and negatively, due to the multifactor nature of this phenomenon.

(v) The possible increase in sediment runoff of unregulated rivers will lead to strengthening of channel processes in deltas. In contrast, this increase will slightly affect dynamics of the marine edge of the deltas. The reason is the expected level rise of the Arctic seas, the strengthening of destructive effect of warm sea waters, waves and currents on the sea coast. Sections of the coast, adjacent to the mouths of large branches, will probably be advanced. Coasts, composed of permafrost rocks and located between the mouths of large branches, will retreat.

(vi) Permafrost degradation will lead to a reduction in the area of thermokarst lakes in the Arctic deltas. This process has already been observed in the north of Western Siberia (Bates et al., 2008; Dneprovskaya & Polischuk, 2008; Kravtsova & Bystrova, 2009).

(vii) It is expected that a sea-level rise in the 21st century will not have serious consequences for the regime and the structure of the Arctic deltas, their ecosystems, population and economy. Sea-level rise will lead to passive flooding of an insignificant part of the delta land, relating to the low-lying delta islands in the seaside area, the beaches on the sea coast, low and young flood plain within seaside reaches of the delta branches, which will strengthen underflooding of the deltas. However, even in low-lying Arctic deltas (Sev. Dvina, Pechora, Pur, Taz, Yana, Indigirka and Kolyma) the width of the coastal land with heights of less than 1 m does not exceed 1–5 km. These areas are not usually developed and not populated, but may have ecological importance.

CONCLUSION

The largest deltas on the Russian Arctic coast are located at the mouths of the Severnaya Dvina, Pechora, Ob, Pur, Taz, Yenisei, Olenek, Lena, Yana, Indigirka and Kolyma rivers. These deltas differ in size, morphology, environment and regime. However, all of these deltas are characterized by cool climate, permafrost, good supply of water resources, dense hydrographic networks, low turbidity of river water, long period of freeze-up, poor land development, uniqueness and vulnerability of ecosystems.

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At present, the Arctic deltas develop under the following conditions: increase in river water runoff; rise of air and water temperature; sea level rise; different vertical movements of the sea coast; and man-induced deterioration of the river water quality. In addition, the hydrological regime of the Yenisei and Kolyma river deltas is subject to water flow regulation by large reservoirs. In these circumstances, the main features of the hydrological regime of the Arctic deltas consist of slow redistribution of water runoff between delta branches, regular inundation of the delta plain and severe ice conditions. Morphological processes are characterized by the relatively low intensity of the channel deformations and delta coast progradation, the prevalence of sediment accumulation in the channel network and delta plain. Moreover, a degradation of permafrost and typical natural landscapes, due to anthropogenic systems is taking place. In the 21st century, the water runoff of the Arctic rivers, sea level, impact of marine factors on delta coasts and regime, the temperature of the air and water will increase. The scale of economic activity in the river basins and deltas will continue to grow, and the quality of river waters may worsen. These changes will have a significant impact on the morphology, regime and environmental conditions of the Russian Arctic deltas. Redistribution of water flow between delta branches will proceed; erosion of delta coasts will intensify; the area of long-term permafrost and area of thermokarst lakes will decrease; the frequency and size of inundation will increase; river and especially sea ice extent and its duration within a year may decrease. Acknowledgements Hydrological regime and the structure of deltas of the Russian Arctic were studied by experts in the Faculty of Geography of Moscow State University (especially V. N. Mikhailov, N. I. Alekseevsky, V. N. Korotaev, R. S. Chalov, D. V. Magritsky, D. B. Babich, A. Yu. Sidorchuk, A. L. Bogomolov and V. I. Kravtsova), the Arctic and Antarctic Research Institute (especially V. S. Antonov, N. P. Gilyarov, V. V. Ivanov, E. P. Kotrehov, A. A. Piskun, Yu. V. Nalimov and D. Y. Bolshiyanov), the State Oceanographic Institute (especially V. F. Polonsky, and U. V. Lupachev), and M. N. Grigoriev, A. P. Lisitsyn, V. V. Gordeev and others. Hydrological and morphological features of Arctic delta evolution are reflected in numerous monographs and articles. The last fundamental monographs (“Geoecological state of the Arctic coast of Russia and safety of environmental management” and “Estuarine-deltaic systems of Russia and China: the hydrological–morphological processes, geomorphology and evolution forecast”), devoted to the river mouths of the Russian Arctic, were prepared in Moscow State University, including with direct participation of authors of the report, and were published in 2007. Research was carried out due to the financial support of the Ministry of Education and Science of the Russian Federation (the contract no. 43.016.11.1628 dated 01.02.2002, no. 02.740.11.0336, no. 11.G34.31.0007 dated 30.11.2010, no. 8342 dated 17.08.2012) and the Russian Fund for Basic Research. REFERENCES Alabyan, A. M., Babich, D. B., Bogomolov, A. L, Zayets, G. M., Korotaev, V. N., Mikhailov, V. N., Sidorchuk, A. Yu. &

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