Boundaries

This ecoregion encompasses the whole Don River basin, including the Severskiy Donets system in the west and the Zapadnyy [Western] Manych system in the east. The ecoregion also includes all of the smaller rivers draining into the Sea of Azov in the west (northward from the Crimean Peninsula). The border between the ecoregion and Dnieper [425] runs along the Dnieper drainage, whereas the border with the Kuban [428] ecoregion follows the lower main course of the river. The Stavropol’skaya Vosvyshennost’ Upland and Ergeni Ridge separate the ecoregion from the Western Caspian Drainages ecoregion [344] to the southeast; the Privolzhskaya Vozvyshennost’ Upland divides the ecoregion from the Volga-Ural ecoregion [341] in the north; and the southern spurs of the Srednerusskaya Vozvyshennost’ Upland separates the ecoregion from Dnieper-South Bug [425] to the west. 

Topography

The Don drainage is situated in the southern half of the Russian Plain between the drainages of the Dnieper River [425] in the west, Volga River [410] in the north and east, and Kuban River [428] close to the Sea of Azov in the south. With a length of 1952 km, and drainage area around 422,500 km², it is the third largest river in the European part of the former USSR, after the Volga and Dnieper. The Don originates in the Central Russian Hills at an altitude of ca 190 m above sea level, and flows into the Taganrog Bay of the Sea of Azov. Throughout the entire extension of the river the right bank is higher than the left bank. Off the right bank, the drainage lies within the Central Russian Hills and its spurs (Eastern Don Ridge), which are considerably dissected. The drainage off the left bank extends through the Tambov Plain, gradually passing into the Volga Hills and Ergeni. Because of the shallow slope, the flow of the majority of rivers within the drainage is slow. The northern part of the drainage receives a larger amount of precipitation than the southern part. It is characterized by a more developed river network, and within the Central Russin Hills is dissected by ravines.

The Severtskii Donets River, the largest tributary of the Don River on its right bank, meets the Don approximately 150 km upstream from Rostov. The length of the river is 1076 km, and its catchment area is 98,660 km². Severtskii Donets originates on the southern slopes of the Central Russian Hills. On the right bank side of the river drainage, relatively short tributaries flow from the Donetskii Range. Like other tributaries of the Don, Severnayi Donets has a low left bank and high right bank considerably dissected by ravines. Throughout the majority of the river the floodplain is covered by numerous lakes, bayous, and backwaters. The width of the river increases quickly, and in the middle and lower reaches is at least 100 m.

Khoper River is the largest left tributary of the Don River, meeting the Don downstream from Veshenskaya stanitsa. It originates on the western slopes of the Volga Hills, and is undulating, with numerous shallows and rifts. Its length is 1008 km, and its drainage area is 61,120 km².

Between the estuaries of the Don and Kuban rivers a number of relatively small streams drain into the Azov Sea, of which the Kagalnik, Yeya, Beisug, and Chelbas are most significant.

Freshwater habitats

The Don River is a typical river, with no large natural lakes in its system. It is fed mostly by snowmelt, which affects the river’s flow regime. The Don freezes on average for 140 days in the upper reaches of the river and for 90 days in the lower reaches (near Rostov). Freezing occurs downstream from the middle of November up to the middle of December; and thawing occurs upstream from middle March to early April. In spring, the high water level of the tributary Severskii Donets may reach, on average, up to 10 m near Kamensk. A large part of the river flow (up to 75 %) occurs during the spring flood (March-April). Average annual expenditure increases from 40.2 m³/sec near the town of Izyum, and up to 200 m³/sec in the estuary. The Severtskii Donets River provides the Don with 6.3 km³ of water annually. Annual flow of the Khoper River is 9.7 km³. The flow velocity of Khoper is low, particularly in the middle and lower reaches of the river. Average annual discharge of the river is 163 m³/sec. During floods it increases on average up to 735 m³/sec, reaching 2700 m³/sec in some years; however, in low-water season it falls to 50 m³/sec.

Rivers of the Azov region are typical steppe shallow streams; their slope is low, current slow, and their river channels are overgrown by reeds in many places. Limans are formed where rivers flow into the Sea of Azov. Flow is observed in spring during snowmelt, and continues for one to two months. In the summer, the rivers become drastically shallow or dry, turning into a number of separate stretches, and their water undergoes salinization. Average annual water flow of the Yeya, the largest river of this group, is approximately 5 m³/sec. The natural regime of the rivers of the Azov Sea has been drastically changed; they have been dammed by dikes, and their waters are used for irrigation and water-supply for settlements. The water content of these rivers has decreased considerably in the past two centuries.

In contrast to the Black Sea, the Sea of Azov contains a low amount of chlorides and an extremely high amount of sulphates, particularly magnesium-containing sulphates (i.e. salt content is similar to freshwater). The Sea of Azov may thus be considered the estuary of the Don River. It is shallow (its maximum depth not exceeding 13.25 m) and hot during the summer, which favors the development of phytoplankton and intensive photosynthesis. At the same time the strong flow of fresh waters that decrease salinity (to 10-12‰) favors the enrichment of water by organic substances. Benthos is extremely abundant, up to 216 g of raw weight per m². Much of the benthos was formed by the mollusks Syndesmia and Cardium, which are consumed by fishes. Thus, a large part of the Sea of Azov is an extensive feeding area for young and adult fishes. The adjoining delta areas of the Don and Kuban rivers are spawning areas of many semi-anadromous and anadromous fishes.

An example of a potamo-mesohaline liman is the Molochnyi Liman, with an area approximately 200 km². Until 1943 it was isolated from the sea by a sand spit. Later, a permanent connection was established between the liman and the sea. As a result, the water level in the liman rose considerably and a gradual ablation of salts from the ground led to desalinization of the liman. Spring floods of the Molochnaya River have freshened the upper reaches of the liman, resulting in salt stratification within the liman. Many limans have been divided by their hydrochemical regime and also by the character of their flora and fauna into an upper, middle, and a lower part.

Beginning 50 years ago, the regulation of the Don and Kuban rivers changed the water regime and salinity of the Sea of Asov. Because of the negative water balance, salinity of the sea in the 1970s increased up to 13-14 ‰, which led to a notable decline in foraging areas, spawning areas, and abundance of many populations. This was aggravated by pressures from illegal fisheries and poaching. In the middle 1980s, the ecosystem was impacted by climatic processes, which resulted in a gradual freshening of the sea. The salinity of the water declined to 10-11 ‰, which conforms with the natural flow regime of rivers of the Azov Sea basin.

Terrestrial habitats

The upper part of the Don drainage is situated in East European forest steppe, and includes trees such as common oak (Quercus robur), littleleaf linden (Tilia cordata), field maple (Acer campestre), and several ash species. The southern part of the ecoregion is located in Pontic steppe, and includes abundant shrublands, thickets, riparian forests, and meadows.

Description of endemic fishes

Don whitefin gudgeon (Romanogobio tanaiticus) was once considered a subspecies of R. albipinnatus. It is distiguished from the sympatric Gobio brevicirris by a shallow, elongated, caudal peduncle, lack of spots on its fins, and epithelial crests on its predorsal scales. It is a benthic gudgeon inhabiting moderately flowing lowland sections of streams. It is locally threatened by pollution and river engineering, as well as the construction of large reservoirs on the Don River.

Danilevskii’s dace (Leuciscus danilewskii) is a medium-size (up to 20 mm) dace that prefers the main course of rivers with clear water, moderate to little current, and a sandy or sandy-clay bottom. It avoids turbid and stagnant waters, although there are records of this fish from Taganrog Bay. This fish may actually be considered an endemic to the Don River drainage. Morphological and molecular data support the distinctiveness of this species from the widespread common dace (Leuciscus leuciscus).

Benthophilus durrelli is a small-sized benthic gobiid that inhabits both fresh and brackish waters. It belongs to the genus, Benthophilus, which is one of the most peculiar fishes – a relict from the Parathetys basin. It is abundant in coastal areas of the Sea of Azov, especially in estuarine regions at river mouths with considerable freshwater, lower reaches of rivers, and dam lakes. The species prefers silty sand with mollusc shells. It occurs in Taganrog Bay of the Sea of Azov, Kuban Delta, and the Don River from the delta to Tsimlyansk Reservoir. It is also found in the Don tributaries, Severskii Donetz, and Manych.

The near-endemic Azov tadpole goby (Benthophilus magistri), a representative of the same genus, has a smaller range and prefers brackish estuarine waters. It mostly occurs in the eastern part of the sea, and only rarely enters the lower reaches of rivers.

Ecological phenomena

The most striking feature of the Don was the historical occurrence of abundant stocks of valuable migratory species – several Acipenser species, Huso huso, Alosa maeotica, Alburnus mento, and some others. The Don supported extensive runs of these species far upstream for spawning. 

Justification for delineation

For ecoregions of the former Soviet Union, a species/genera/family presence/absence matrix was compiled for a hierarchy of hydrographic units, and cluster analysis and ordination techniques (Primer v.6 statistics software) were employed to assess biotic similarities among hydrographic units and to identify major faunal breaks.

This ecoregion is composed of the Don River and smaller rivers off the coast of the Sea of Azov. These smaller rivers lack populations of larger migratory and semi-migratory species, although they possess the main riverine sedentary forms. Being the largest lowland river of the Sea of Azov basin, the Don had once exclusively supported numerous stocks of semi-anadromous and anadromous species that foraged in the Sea of Asov and Taganrog Bay, and spawned in the Don. Historically, the Don had many more spawning sites for migratory sturgeons and herrings than the Kuban River, which also flows into the Sea of Azov. The Don is distinguished, in general, by the absence of native Salmoniformes and Barbus. Besides this, it differs from Dnieper - South Bug [425] by a different species composition of the genera Benthophilus, Gobio, and Romanogobio, and lack of Neogobius kessleri.

Level of taxonomic exploration

Good