In its upper reaches, the Yenisey is a mountainous river with rocky shores and rapids; from below Krasnoyarsk to Dudinka, it forms a wide valley that divides the West Siberian lowland from the Central Siberian plateau. The main tributaries are the Angara, Podkamennaya Tunguska, and Nizhnyana Tunguska. The river freezes over between late October and early June in the lower reaches, and between mid-November and late April in the upper reaches. It is chiefly used for the transportation of timber, grain, and coal, and is navigable by sea-going vessels as far as Igarka. Hydroelectric power stations have been built along its course. The Yenisey region was first explored by Russia in the 16th century and its basin colonized from the 17th century. Forced labour camps were a feature of this bleak region during the Soviet period (see Norilsk).