The European part of the Soviet Union had extensive, highly developed, and heavily used water-resources, among them the key hydrosystems of the Volga, Kama, Dnepr, Dnestr, and Don rivers. As with fuels, however, the greatest water resources lay east of the Urals, deep in Siberia. Of the sixty-three rivers in the Soviet Union longer than 1,000 kilometers, forty were east of the Urals, including the four mighty rivers that drain Siberia as they flow northward to the Arctic Ocean: the Irtysh, Ob', Yenisey, and Lena rivers. The Amur River formed part of the winding and even at times there are tense boundaries between the Soviet Union and People's Republic of China. Taming and exploiting the hydroelectric potential of these systems became a monumental and highly publicized Soviet project. Some of the world's largest hydroelectric stations operated on these rivers. Hundreds of smaller hydroelectric power-plants and associated reservoirs were also constructed on the rivers. Thousands of kilometers of canals linked river- and lake-systems and provided essential sources of irrigation for farmland.