Селенга
The Selenga River basin is part of the larger international Arctic Ocean Basin and is located in northern Mongolia. A semi-arid region, the basin is 280,000 km2 and is Mongolia's largest basin.  The basin has two main rivers: the Selenga and the Selenga's tributary, the Orkhon.  The "major sub-basins are the Egiin, Ider, Orkhon and Tuul river basins" with Lake Baikal occupying 47% of the Selenga River basin. The terrain of the basin is dominated "by forest or grassland and is underlain by permafrost."