a river in Tiumen’ Oblast, RSFSR. A left tributary of the Ob’, the Bol’shoi Iugan discharges into the Iugan channel. It is 1,063 km long, drains a basin of 34,700 sq km, rises in the swamplands of Vasiugan’e, and flows through the West Siberian Plain. Its chief tributary is the Malyi Iugan (on the right). In its basin there are approximately 8,000 lakes, with an overall area of 545 sq km. It is fed primarily by snow. The Bol’shoi Iugan freezes over in October or November and opens up again in late April or early May. Logs can be floated down the river. It is navigable below the Kuplandeev’s harbor.