Originating from its headwater tributaries, the Ider and the Delger mörön, it flows for 992–1,024 kilometres (616–636 mi)[2][3][4] before draining into Lake Baikal.

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.

River modifications like the Irkutsk Hydroelectric Power Station have affected the environment of the Selenga Delta, causing it to become waterlogged.

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.

An alternate source believes the name originated with the Evenki word sele ("iron") to which the possessive nge suffix was added.

[6] 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.

[6] In this expedition, Sosnovokii discovered remains of local Paleolithic culture in an area that stretched from "the valley of the Selenga River from the Mongolian Frontier down to Selenginsk.

[6] Directed by A. P. Okladnikov, the study uncovered 30 new sites, including one that covered the area from Selenginsk to the Selenga River mouth, which provided evidence of a Paleolithic-type culture.

[7] In a study published in volume 181 of the journal Cell, the link between Native Americans and their ancestors in East Asia was established using one of the tooth fragments found at the Ust-Kyakhta-3 site.

[10] The flood did not heavily impact settlements on the banks of the Selenga River, but there were reports of bridge and telegraph pole destruction between the towns of Selenginsk and Verkhneundinsk.

[10] The volume of water inflow into Lake Baikal during the 1897 flood is estimated to have been approximately 22.2 cubic kilometres (5.3 cu mi).

[12] Due to the anti-cyclone, the 448,000-km2 river basin is prone to a climate of harsh winters,[11] which consist of sunny days and low air temperatures,[12] and warm summers.

[12] For 80–90 days, starting in mid-June, the basin area experiences the summer season with precipitation totaling 250–300 millimetres (9.8–11.8 in).

[14] The terrain of the basin is dominated "by forest or grassland and is underlain by permafrost" according to an article published in 2003 by Ma et al.[15] Lobate shaped and 40 km wide, the Selenga River delta is surrounded by marshlands.

The formation of the delta is dependent on the sediment load carried by the river, with the lake tides having little impact in terms of geographical change.

[16] The sediment balance of the delta is dependent on physical processes, and this, in turn, affects the geochemical barrier functions of the area.

[13] The region includes the Orkhon and Selenge river basins, and features scattered forests as well as extensive mountain ranges.

[13] The area has a dry and humid climate that is influenced by warm winds from the northeast steppe regions.

[20] In 2015, Mongolia was reported to be planning the construction of a dam, named the Shuren Hydropower Plant, on the Selenga River.

[21] 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.

[21] Disrupting the flow of the river would disturb the breeding grounds of many endemic fish species in Lake Baikal.

[21] The Shuren Hydropower Plant Project was proposed in 2013 and was under the review of a World Bank-funded environmental and social impact assessment.

[23] The first migration period begins in the second half of April when the water temperature is 3-5 degrees Celsius and finishes in approximately mid-June.

[23] Large populations of migrating, breeding and moulting waterbirds assemble at the wetlands in the Selenga Delta,[24] which has been recognised as an Important Bird Area by BirdLife International.

[12] Since the collapse of the Soviet Union in the 1990s, the basin, which contains vast plains, has been affected by agricultural, climatic and economic factors.

[9] A study published in volume 12 of the journal Water, found that based on the Mongolian National Standard, "the main pollutants of the Selenga River are Zn ... As ... and Pb ..."[12]  They also found that metal content in the river water was tens or hundreds of times higher in sites closer to urban and mining areas.

[12] Based on data observing toxicological and bioaccumulation effects on aquatic biota, the contamination has been shown to have an ecological impact.