The Selenga River (Selenge River, Mongolian: Сэлэнгэ мөрөн, Selenge mörön; ; ) is a major river in Mongolia and Buryatia, Russia. Its source rivers are the Ider River and the Delgermörön river. It flows into Lake Baikal and has a length of or , according to other sources. The Selenga River is the headwaters of the Yenisei-Angara River system. Carrying of water into Lake Baikal, it comprises almost half of the riverine inflow and forms a wide delta of , when it reaches the lake.