Селенга
After the Russians reached Trans-Baikalia in the 1640s, some trade developed, but it is poorly documented. At this point there were three routes: 1) Irtysh River and east across Dzungaria and Mongolia, 2) Lake Baikal, Selenga River and southeast (the shortest) and 3) Lake Baikal, east to Nerchinsk, and south (slow but safe).