Селенга өзені: Моңғолия мен Ресейдегі маңызды артерия

River in Mongolia and Russia

The Selenga or Selenge (/s//ɛ//l//ɛ//ŋ/'/g//ɑː/ ; ; Селенга́, ) is a major river in Mongolia and Buryatia, Russia. Originating from its headwater tributaries, the Ider and the Delger mörön, it flows for before draining into Lake Baikal. The Selenga therefore makes up the most distant headwaters of the Yenisey Angara river system. Carrying of water into Lake Baikal, it makes up almost half of the riverine inflow into the lake, and forms a wide delta of when it reaches the lake. Periodic annual floods are a feature of the Selenga River. The floods can be classified as “ordinary”, “large” or “catastrophic” based on the degree of impact. Of the twenty six documented floods that occurred between 1730 and 1900, three were “catastrophic”. The three “catastrophic” floods were the floods of 1830, 1869 and 1897. The Selenga River basin is a semi arid region that is in area. It is part of the Arctic Ocean Basin and is located in northern Mongolia. Stone implement artifacts found on the Selenga River at the end of the 19th century have been used to form a link between Native Americans and their ancestors in East Asia. The climate of the Selenga basin is influenced by a powerful Siberian anti cyclone. The weather consists of harsh winters and warm summers, with the river freezing from November to April. The Selenga River delta is a Ramsar site, surrounded by marshlands. Found in the administrative region of the Republic of Buryatia, the wetlands are the habitat of a large number of threatened and endemic species, including the Siberian Baikal sturgeon and over 170 species of birds. The Selenge Orkhon forest steppe includes the areas of the Orkhon and Selenge river basins. The region has scattered forests and extensive mountain ranges. River modifications like the Irkutsk Hydroelectric Power Station have affected the environment of the Selenga Delta, causing it to become waterlogged. Activities including mining, agriculture, breeding and dumping of wastewater have also affected the environment of the river. There has been an observed degradation of water quality from these anthropogenic factors. Natural factors like erosion processes and weathering have also contributed to the decline in water quality. The proposed Shuren Hydropower Plant Project was protested against by environmental groups because of the potential ecological ramifications it would have on the Selenga Delta. The reasons cited included potential disruption to the flow of the river and the breeding grounds of endemic fish species in Lake Baikal.

The name Selenge comes from Mongolian seleh, which means "to swim". 'Selenga' is the russified version of the same. An alternate source believes the name originated with the Evenki word sele ("iron") to which the possessive nge suffix was added. The Selenge Province of Mongolia is named after the river.

At the end of the 19th century, evidence of Paleolithic culture was discovered in the Selenga River basin. The artifacts found were collections of stone implements. During 1928 and 1929, G. P. Sosnovskii, under the purview of the Academy of Sciences of the U. S. S. R., directed an archaeological Stone Age study in the Selenga Valley near Kiakhta. Further archaeological work in 2012 at the site found two human tooth fragments.

Periodic floods are characteristic of the Selenga River, with small flooding events observed annually. The floods of the Selenga River can be classified based on the degree of impact. It flows from Mongolia and into east central Russia, acting as the main tributary of Lake Baikal. Due to the anti cyclone, the 448,000 km2 river basin is prone to a climate of harsh winters,  This is in contrast to autumn and winter, in which rainfall is typically In mid September, autumn begins, lasting for about 65 days. A semi arid region, the basin is 280,000 km2 and is Mongolia's largest basin. The sediment balance of the delta is dependent on physical processes, and this, in turn, affects the geochemical barrier functions of the area. During the period of the Irkutsk dam construction (1950–56), there was "increased sediment delivery to the delta" according to an article published in 2017 by Chalov et al. The wetlands of the Selenga River delta are classified as a Ramsar site and are found within the administrative region of the Republic of Buryatia. The wetlands support a large number of threatened and endemic species, including over 170 species of bird. Built on the Angara River, the station was designed to raise the water level of the Angara River to match the water level of Lake Baikal. Environmental groups protested this plan, as they were concerned with the potential ecological ramifications, building a hydroelectric plant on the Selenga River, would have on the ecosystem of Lake Baikal.

Leocottus kesslerii gussinensis and Leocottus kesslerii arachlensis are endemic subspecies of fish that are found in the Selenga River headwaters and the upper Khilok River. Lake Baikal is the habitat of the Siberian Baikal sturgeon.

Siberian crane, Grus leucogeranus Anthropogenic factors include mining activities, agriculture, breeding, dumping of wastewater into water bodies and water use.

The Selenga River has an influential role in managing the economic futures of Mongolia and Russia. These wastewater treatment plants are located in the Mongolian cities of Tsetserleg, Bulgan, Darkhan City, Uliastai, Erdenet City, Arvaikheer, Sukhbaatar, Zuunmod, Murun and Ulaanbaatar. The city of Ulan Ude uses the Selenga River as its primary source of water for its municipal water supply, using 60 wells with submersible pumps to withdraw water. The total pumped water in 2009 was .