The Nenjiang or Nen River ("Nonni" in Russian) originates in the low hills that define China’s northeastern border with the Russian Far East. The Nen River basin is enclosed by low mountains, the Great Hinggan to the west and the Small Hinggan to the north and northeast. Higher mountains that form the base of the Korean Peninsula define the southern margin of the plain. The whole region drains into the Sea of Japan through the Songhua River.
Soils in the center of the basin have been partly deposited from rivers and lakes throughout the Quaternary period. These tend to be poorly drained, creating swampy, sometimes saline conditions in the low-lying areas, some of which have boggy peat soils. Westward, this swampy landscape undergoes a transition to the drier steppes of the Great Hinggan foothills. Eastward, near the Small Hinggan foothills, one finds a wide margin of famous rich black-soils (Chernozems) developed under forest-steppe vegetation. These soils, once supporting forest-steppe ecosystems have been almost completely converted to crop cultivation.
The Nen River has a continental monsoon climate and is warmer and drier than the surrounding mountains. Mean annual precipitation is 400 to 450 mm.
Typical landscapes are flooded meadows and shallow, reed-filled lakes, rivers and old river courses undergoing ecological succession to grassland. Lakes may be either fresh or brackish, and salt concentrations are increasing in many areas as a result of freshwater diversions for irrigation. Meadows are dominated by grasses such as (Calamagrostis epigeios) and (C. langsdorfii)that are adapted to flooded soils. These often grow as dense tussocks that emerge from flooded areas. Lakes are often filled or lined at the margin by the salt-tolerant reed, Phragmites communis. Upland areas are dominated by grasslands, forest-grasslands with crooked elms, and shrub groves of wild-apricot on the hilltops.
During the April-June breeding season, productivity is high, with abundant fish, frogs, mollusks, and aquatic insects, making this an ideal breeding area for waterfowl. More than 200 bird species have been recorded here, including at least six of the world’s 15 crane species. The three species that breed here include red-crowned crane (Grus japonensis, IUCN Endangered), white-naped crane (G. vipio, IUCN Vulnerable), and Demoiselle crane (Anthropoides virgo). Three species stage here prior to migrating to their breeding habitat, the common crane (G. grus), Siberian crane (G. leucogeranus, IUCN Critically Endangered), and hooded crane (G. monacha, IUCN Vulnerable). Other rare bird species that breed here are Oriental White Stork (Ciconia boyciana, IUCN Endangered), Black-headed Ibis (Threskiornis aethiopicus melanocephalus), Mandarin Duck (Aix galericulata), and Eurasian Spoonbill (Platalea leucorodia). Common amphibians include toad (Bufo raddei), tree frog (Hyla arborea), frogs (Rana nigromaculata and Rana amurenss).
Habitat degradation is caused by agriculture, fish farming, reed harvest, hunting, and collecting bird eggs. Salinization has become a problem in many areas as well. This occurs when demand for irrigation water is so high that insufficient fresh water passes through the system to thoroughly flush salts. However, direct desiccation of wetlands due to water diversion for agriculture, flood control, oil industry, municipal, and other needs is the largest problem. Wetlands in many nature reserves experience extended drought coupled with increasing frequency and acreage of wildfire. During the past 100 years the Nen River valley, the largest grassland-wetland of the Amur-Heilong basin, has been reduced to a series of isolated and shrinking wet grass habitats.
Numerous nature reserves have been established to protect the remaining Nen River valley wetlands: Zhalong Nature Reserve (2,100 km2), Momoge Nature Reserve (1,440 km2), Xianghai National Nature Reserve, Ke’erqin National Nature Reserve, Tumuji National Nature Reserve (1,000 km2), and others. Even so, there is no reliable regulatory mechanism to ensure sufficient and timely water supply to these wetlands. This leads to degradation of habitat due to desiccation and agricultural encroachment.