Boundaries

The ecoregion is defined by the widened bed of the Middle Amur [Heilong Jiang = Heilungkiang] from the confluence of the Shilka and Argun’ rivers to a point 420 km downstream from Blagoveshchensk where the Amur cuts the mountainous ridges of the Malyy Khingan (in the south) and the Bureinskiy Khrebet Range (in the north) and enters the wide Lower Amur plain.

In the north (downstream from the confluence of the Argun’ and Shilka rivers), the border runs along the northern spurs of the Bol’shoy Khingan Range [Da Hinggan Ling], which separates the Haylar [Hailar He] drainage (Argun’ ecoregion [618]). The Malyy Khingan Range [Xiao Hinggan Ling] divides the Sungari [Songhua Jiang] drainage (ecoregion 620) and the right-hand tributaries of the Middle Amur. The left-hand tributaries of the Middle Amur have much wider drainage areas that are delimited in the north (from west to east) by the Amazarskiy Khrebet Range (from the Shilka River [619]), Olekmenskiy Stanovik Mountains (Cheremnoy and Dzheltulinskiy ranges), Stanovoy Khrebet Range (from the Aldan River headwaters, 608), Dzhugdyr and Dzhagdy ranges (from the Uda River drainage area [615]), and Yam-Alin’, Dusse-Alin’, and Bureinskiy Khrebet ranges (from the Amgun’ River drainage [616]).

Topography

Downstream from the confluence of the Shilka and Argun rivers, the Amur flows through mountainous terrain. Between the Nyukzha (in the north) and Bolshoi Khingan (in the south) ridges, the Amur passes between high rocky coasts and has high flow velocities. After the confluence with the Kumara River the Amur River enters a broad valley. Near Blagoveshchensk it receives the Zeya on the left. Downstream from Blagoveshchensk the river flows mostly through a broad valley in a channel, edged by low swampy coasts in some parts; the mouth here is split into numerous anabranches. The Bureya (about 640 km in length) falls into the Amur 270 km downstream from the Zeya. Downstream from the Bureya, at the cross with the Malyi Khingan, the Amur valley narrows abruptly, and its waters gather into one full flow (around the boundary with the Lower Amur [616]).

The Zeya is one of the largest rivers of the Amur catchment. It flows into the Amur from the left (near Blagoveshchensk), is 1210 km in length, and has a drainage area of 233,000 km². The Zeya originates on the southern slopes of the Stanovoi Ridge. In the upper reaches, from the source up to the estuary of the Selemdzhi, it has a mostly mountainous character. Here, its valley is restricted by high slopes. Where it crosses the Tukuringir Ridge the river flows into a deep rocky ravine. In the lower reaches (downstream from the confluence with the Selemdzhi River) the Zeya River spreads out onto the plain, where its valley extends and the channel is split into many anabranches. The large Zeya Reservoir was built in the upper reaches of the Zeya River.

The Bureya is the second largest left tributary of the Amur River. It originates on the northern slopes of the Bureinsky Ridge, is 716 km in length, and its drainage area is about 70,000 km². The Upper Bureya, approximately up to the Paikan Station, is mountainous in character; banks here are rocky in some areas, and the current velocity can be 2 m sec ^-1 or more. In its lower reaches the Bureya River flows beyond the limits of the Zee-Bureinskaya plain. Where the valley extends, the channel is restricted by low coasts and is divided into branches and anabranches, forming numerous islands. The Bureya is one of the largest rivers of the Far Eastern Region; its average annual water flow is equal to 950 m³ s^-1. Its main tributary, the Tyrma River, is 313 km in length and the drainage area is 15,200 km².

The Middle Amur River drainage contains no large lakes, and the small ones are concentrated mainly in the catchment of the Zeya River. There are mountain lakes of the upper reaches of the Zeya River; lakes on the slopes of the Ezop and Dusse-Alin ridges (drainages of the Selemdzha and Bureya rivers), at elevations of 1200-1500 m; lakes of elevated plains and intermountain hollows, including numerous lakes of the Verkhnezeyskaya and Verkhnebureinskaya hollows (elevation 300-400 m); lakes of the Zeisko-Selemdzhinskaya foothills plain (elevation 250-300 m); and lakes of the Zeisko-Bureinskaya plain (elevation 100-200 m). In general, elevations in the ecoregion range between 60 to over 2000 m above sea level.

Freshwater habitats

Rivers of the ecoregion have high flow, with an average annual water flow of 1800 m³/sec. Melting snow and summer rains commonly result in catastrophic flooding. In winter, the Zea and Bureya rivers contain little water since they flow through the permafrost zone. Fluctuations in water levels can reach 9-10 m.

Freezing-over lasts about six months. Because of the low temperatures and little snow in winter, the thickness of the ice is commonly very high, and often frazil ice appears. Spring ice is sometimes accompanied by massive ice jams.

The natural hydrological regime of rivers in this ecoregion has been substantially modified by the construction of the Zeskaya Dam and reservoir.

Terrestrial habitats

The mountainous part of the ecoregion is much larger than the lowland part, which is often marshy. Terrestrial ecoregions include Da Hingann-Dzhagdy Mountains conifer forests, Amur meadow steppe, Manchurian mixed forests, Okhotsk-Manchurian taiga, East Siberian taiga, and Trans-Baikal Bald Mountain tundra.

Description of endemic fishes

Thymallus burejensis is known only from the Bureya River. It is a peculiar grayling with a restricted range.

Other noteworthy fishes

Amur grayling (Thymallus grubii) was once commonly considered to be a single species in the Amur drainage system. However, a number of publications revealed that the range of the Amur grayling is limited by the headwaters of the Amur down to the confluence with the Bureya River.

Ecological phenomena

This ecoregion lacks runs of most Amur migratory fishes, but it previously supported the run of the "autumn" keta (Oncorhynchus keta), which migrated 2000-3000 km upstream from the sea. This stock has been extirpated.

Justification for delineation

The ecoregion is defined by the bed of the Middle Amur from the confluence of the Shilka and Argun’ rivers to a point about 420 km upstream from Khabarovsk (at Ekaterino-Troitsk) where the Amur Rivers cuts the mountainous ridges of Maly Khingan and enters the wide Lower Amur plain (the border with ecoregion 616). It includes the catchments of the largest northern tributaries of the Amur – the Zeya and Bureya rivers. It is characterized largely by mountainous and hilly areas, and, accordingly, fast-running water over stony bottoms. The climate is severe, especially around the headwaters of the Zeya and Bureya rivers. This determines the composition of fish assemblages from poor in the upper sections to relatively diverse in the main course of the Amur River. This ecoregion lacks a number of species found in the Lower Amur [616].

Level of taxonomic exploration

Fair: data on the distribution of many species are based on isolated findings.