The damming of rivers produces dramatic and far-reaching environmental changes: of the first order are hydrological changes, including water quality; the second order affects channel morphology and aquatic, riparian, and floodplain vegetation; and the third order includes vertebrates and fish. Thus there are important linkages between the physical, chemical, and biological components of river systems that are disordered when a river is impounded.
The most significant changes take place in the hydrological regime which, in turn, affects many other natural processes. Stream flow is most influenced by reservoirs with long-term and marked seasonal regulation; the upstream levels can change over a year by amounts ranging from a few tens of centimetres to tens of metres and in some reservoirs by over a hundred metres. Marked variations in level and water exchange rate affect the flow, temperature, and hydrochemical and hydrobiological regimes of reservoirs and downstream rivers as well as silting and changes in beds and banks. The flow of suspended and dissolved materials, including mineral biogenic substances and trace elements, also drops sharply.
Reservoirs cause substantial changes in ground water. Depending on the local hydrogeological conditions, the range over which the ground-water head spreads can vary from tens of metres to many kilometres. In some cases this is intended and considered favourable but where substantial vertical and horizontal spreading of the ground-water head swamps can be created and land flooded.
Reservoirs greatly influence the landscape of the river valleys, including the proportion of land to water area as well as the drainage network pattern. Changes in the landscape are also a result of changes in soils, vegetative cover and wildlife. The areas, volumes, widths and lengths of the reservoirs vary markedly over a year; the strips of land that are regularly drained may become partially covered with vegetation, or turn into sandy or silty shoals. Downstream flooding of the plains decreases or ceases altogether, which results in the former channels and flood lakes becoming dry, silted up and overgrown. Downstream channels and flood plains can change their shape, through erosion of the banks and bottom and drying up of flood-plain branches and mainstream channels. Transformation of the channels usually continues until equilibrium is reached between degradation and aggradation. The erosion zone gradually moves downstream. Channel erosion leads to a lowering of the water level. At flood time, because of the lower high-water level in the main stream, the current velocity in the tributaries rises and the erosion of their beds and banks increases. Some alterations also occur in the downstream flow and delta. The flow is redistributed between the branches in favour of bigger and deeper ones, which results in silting and drying up of the smaller branches and erosion of the bigger ones.
The creation of large reservoirs changes the climate, soils, vegetation and fauna. The area affected depends on local factors, the dimensions and operating regime of the reservoir and its geographical position. The main alterations in the meteorological conditions are as follows: radiation and evaporation increase, the climate becomes less continental, wind speeds rise, and breeze-like winds appear. Downstream of the water installations, winter discharges increase, and their influence on the air temperature and humidity is stronger. The thermal conditions of the water in cold climates changes considerably: winter temperatures are substantially higher, air humidity is higher and fogs may occur.
The mode and intensity of the reservoir's effects on the soil and vegetation depend upon the ground-water regime, the lithological composition of the soil, the mode of regulation and changes in the micro-climate. The ares of shore affected around large reservoirs may vary from hundreds of metres to several kilometres. Changes in the downstream soil and vegetation are not so marked as at the reservoirs. Less, if any, fertile mud is deposited on grasslands, the amounts of moisture stored in the ground of flood plains are reduced, and the ground-water level is lowered. In the majority of river valleys, the soil and vegetation zones shift down towards the river channels.
Reservoirs cause substantial changes in the wildlife over wide areas. During filling, the number of many species of animals and birds is considerably reduce, as their habitats and feeding areas are disturbed. In mountain regions, big reservoirs disturb the migration paths of hoofed animals. During the operation of reservoirs, the habitat conditions are greatly affected by the water level variations. Because of changes in the shores, numerous animals and birds are deprived of their habitats. On many reservoirs, the amount and, sometimes, the number of species of waterfowl increase. Over time a balanced biocoenosis gradually develops.
During the first years after the reservoir starts operation, the water quality deteriorates owing to organic and mineral matter being washed out from the bottom and shores, decay of vegetable residues and the negative influence of peat. The downstream quality of water depends directly on how much water is released and how often. As a rule the amount of nutrients entering the water is reduced. As compared with natural watercourses, reservoirs are, as a rule, less capable of self-purification; for example, algal blooms may become more frequent.
Conflicting with contact uses of water in reservoirs, particularly in the tropics, have been water-borne or water-organism-vectored diseases and invasion by aquatic nuisance plants. Urinary schistosomiasis should now be considered an expected impact of dams constructed in endemic areas. The explosive growth of aquatic weeds and the establishment of a wide range of invertebrates, among them the intermediate hosts of human parasites, lead to the spread of schistosomiasis and other public health problems in a dam area. Domestic water supply and waste disposal are seldom adequate in most resettlements and in such conditions, with the immigration of infected people to the dam area where there is constant contact with water containing vector snails, schistosomiasis soon becomes established and quite often exceeds onchocerciasis and malaria in severity. These health problems are among the most severe of the impacts directly affecting people associated with dams.
Fish catch from the Mediterranean sea adjacent to the Nile delta decreased from 38 000 tons in 1962 to 14 000 tons in 1968, as a result of the Aswan dam construction that reduced nutrient and sediment exports.
Around the world, the answer to the increase in water demand has been to build more environmentally destructive dams and divert more rivers. The number of large dams worldwide has climbed from just over 5,000 in 1950 to 38,000 in 1999.
The question of the merit or otherwise of constructing a reservoir has no simple answer. Dams large and small are necessary for development.