a river in Irkutsk Oblast, RSFSR, the right tributary of the Lena. It is 460 km long, drains an area of 26,900 sq km, and flows in a deep canyon along the Patom Highlands. It is full of rapids. Fed primarily by snow and rain, the Bol’shoi Patom freezes over in October and opens up again in May. Timber can be floated down its lower reaches.