The northward-flowing rivers of Siberia deliver immense quantities of water but only relatively small quantities of sediment to the Arctic Ocean. The relatively low delivery of sediment to the ocean by these rivers is explained by the large areas of forest and swamp in their basins. In the Ob' River, sediment yields tend to increase between the headwaters and Kamen' na Obi. Further downstream, sediment yields gradually decrease. Near the mouth of the Ob' River, at Salekhard, sediment yield has remained constant at about 5.3 t km-2 year-1. In the lower Yenisey River, the already small natural sediment yield of 5.4 t km-2 year-1 has been decreased several fold by the construction of massive reservoirs on the main stem and on the tributary Angara River, and presently equals 1.8 t km-2 year-1.