With its headwaters in Mongolia, the Selenge River flows about a thousand kilometers to empty into Lake Baikal, and supplies the lake with about half of its inflow, according to the above link. Here's the Flash Earth link.
The river that drains Lake Baikal is the Angara, which is a major tributary of the Yenisey River, which, in one of those delightful coincidences that keep life surprising, I am already familiar with.P. Kyle House, PhD., Surficial Geologic Mapping Specialist / Geomorphologist