The Ust-Ilimsk Dam on the Angara (downstream from the older, bigger, and better known Bratsk Dam), completed in the mid-1970s, not only backs up the Angara, but also the Ilim as far as Zheleznogorsk-Ilimsky. The site of the old town of Ilimsk was flooded by the reservoir.
The Ilim is 589 kilometres (366 mi) long, and its basin covers 30,300 square kilometres (11,700 sq mi). The river freezes up in late October and stays icebound until early May. Its main tributaries are the Kochenga, Tuba, and Chora Rivers.