a river in Irkutsk Oblast, RSFSR; right tributary of the Angara River. Length, 589 km; basin area, 30,300 sq km. The Ilim rises in and flows over the Central Siberian Plateau. From its source to the village of Ilimsk it is a mountain river. The Simakhinskii Rapids, which are formed by traps, are 8 km from the mouth. The river is fed by snow and rain. The average discharge at the populated point of Sotnikov (52 km from the mouth) is 139 m³/sec.

The Ilim freezes over at the end of October and opens up in early May. Its main tributaries are the Kochenga and the Tuba on the right and the Chora on the left. It is suitable for floating logs and is navigable for small vessels as far as the village of Ilimsk. The Ust’-Ilimsk Hydroelectric Power Plant is under construction 40 km below the mouth of the Ilim, on the Angara River. The Korshunovskoe iron ore deposits are located in the basin of the Ilim. The so-called Lena Portage during the 17th to 19th centuries passed through the village of Ilimsk on the Lena River by way of the Muka and Kuta rivers (Lena basin).