The River of Russia is a vast network of rivers that span the country. The Ural River is the longest river in Russia, stretching 2, 428 miles from its source in the Ural Mountains to its mouth in the Caspian Sea. The Amur River is the second longest river in Russia, stretching 2, 824 miles from its source in the mountains of the Russian Far East to its mouth in the Sea of Okhotsk. The Seim River is a tributary of the Amur River, originating in the mountains of the Russian Far East and flowing for 845 miles before joining the Amur River.