The Selenga River is Mongolia's principal waterway and forms the main tributary to Lake Baikal in southern Siberia near Russia’s border with Mongolia. The river is formed by the confluence of the Delger and Ider Rivers and is about 600 mi (1,000 km) long. The Delger rises in the Sangilen Mountains on the border between Mongolia and the Russian Republic of Tuva, while the headwaters of the Ider lie in the Hangayn Mountains in central Mongolia. The Selenga, along with the Upper Angara and the Barguzin Rivers, provides one of the main inflows to Lake Baikal. The mean discharge of the Selenga River into Lake Baikal in winter is approximately 100 m3/s which increases to 1,700 m3/s in spring as the snow melts. A multibranch fan-like channel structure forms the Selenga Delta.