Yenisei (yĕn′ ḗ-sā′ ḗ), one of the largest rivers of Siberia, formed by the junction of the Onlon-Keme and the Bey-Keme, which rise in the mountains on the southern border of the country. It flows northward through the center of Siberia into the Arctic Ocean, forming at its mouth a long estuary. In the earlier part of its course it is interrupted by falls and rapids, but afterwards flows through a great plain or steppe, receiving many tributaries, its total length being over 3,000 miles.