For a long period obtaining energy from renewable sources was favoured by specialists as the method that is least invasional to environment. Energy that is obtained from dammed rivers belongs to such ecological sources. The time of maximum intensity of impounding rivers was till the 60s of the 20th century when profits of retention and other benefits, including economical considerations, dominated. At present there are two trends: 1) to build new huge reservoirs like the China Three Gorges Dam of the Yangcy River, 2) to liquidate some reservoirs (dam removal). The second trend is partially a result of shallowing of reservoirs and partially of hydrobiologists’ investigations. From 1980 (Ward and Stanford) ecologists have been stressing the negative influence of dams on river biocenoses. The strongest influences on riverine organisms‘ biology are exerted by changeable flow regime and water temperature. Dams also reduce the migrations of both fish and macroinvertebrates, which ensue from interrupting the river continuum. Because, due to their aging, reservoirs possess the predicted time of their functioning (about 100 years), in some countries (USA) the trend of dam removal has started. It concerns mostly small and mid-sized reservoirs. Also, other methods of improving rivers have occasionally been implemented (for example application of artificial floods on the Colorado River, downstream of the Glen Canyon Dam). Thus water energy, produced by impoundments, constitutes a considerable threat to nature.