River Siberia is a major river system in Russia, consisting of the Lena and Amur rivers. The Lena River is the longest river in Siberia, stretching over 4, 400 km from its source in the Baikal Mountains to its mouth in the Laptev Sea. The Amur River is the second longest river in Russia, flowing for over 2, 800 km from its source in the mountains of the Russian Far East to its mouth in the Sea of Okhotsk. Together, the Lena and Amur rivers form the backbone of the River Siberia system.