Since Russia is the largest country of all in terms of area, no wonder that it is the country that has many of the world's longest rivers flowing through its territory. The longest of those is the Yenisei-Angara-Selenga system, with a total length above 5500 kilometers, it is the longest river that drains onto the Arctic Ocean. The Selenga river has its source in Mongolia, then enters Russia south of Lake Baikal, and flows into the deepest lake on Earth as well. The Angara river drains Lake Baikal, and connects it to Yenisei River after traveling almost 1800 kilometers in Siberia.
The second longest river system is the Ob-Irtysh system. The Irtysh river originates on the Chinese-Mongolian areas of the Altai Mountains, then cuts through northeastern Kazakhstan before entering Russia, and joining the Ob river upper in the north. The Ob river empties into the Kara Sea, not far from the mouth of the Yenisei river.