While wide variations occur from site to site, the environmental impacts of dams can generally fit within two categories: those due to the existence of the dam and reservoir; and those due to the pattern of dam operation (Table 1).

A. Impacts due to existence of dam and reservoir:

1. Upstream of dam, the imposition of a reservoir in place of a river valley.
Dams have flooded a vast area of land -- approximately 400,000 sq km worldwide. This has included terrestrial and river ecosystems of every habitat type, including a considerable tropical forest habitat, particularly in Latin America and Southeast Asia, with its particularly rich biodiversity. Especially significant is the fact that the land lost is of
importance out of proportion to its size, as it includes river habitats, and terrestrial habitats on floodplains and along banks of rivers, that are often among the most diverse ecosystems in world. These habitats are replaced by a relatively uniform reservoir, which will usually provide habitat for a much smaller range of species.

2. Changes in downstream morphology of riverbed and banks, delta, estuary and coastline due to altered sediment load.
Much of the impact of dams on downstream habitats is through changes in the sediment load of the river. All rivers carry some sediment as they erode their watershed. When the river is held behind a dam in the reservoir for a period of time, most of the sediment will be trapped in the reservoir, and settle to the bottom, so that water released by
the dam will be much more clear, with less sediment than it had once had. For example, before the High Aswan Dam the Nile carried an average of 124 million tons of sediment to the sea each year, and deposited another 9.5 million tons on the floodplain; now, 98% of
the sediment goes to the bottom of the Nasser Reservoir. Clear water below a dam is said to be "hungry": it will recapture its sediment load by eroding the downstream bed and banks. Eventually, all the easily erodible material on the riverbed below the dam will be eroded away, leaving a rocky streambed, and a poorer habitat for aquatic fauna. Erosion may also increase along the coast beyond the mouth of the river, as observed, for example, downstream of the Akosombo Dam in Ghana. Overtime, the downstream river will also tend to become narrower and deeper, which will also reduce the diversity of animal and
plant life that it can support.

3. Changes in downstream water quality: effects on river temperature, nutrient load, turbidity, dissolved gases, concentration of heavy metals and minerals.
When river water is held in a reservoir for a period of time, the quality of the water is affected in several ways: the temperature changes, nutrients are removed, forests are flooded and decompose, and there may be colonization of the water by aquatic plants, for example. Each of these effects may have an impact on the life that depends on that water. These effects are generally related to how long the water has remained in the reservoir. Particularly severe effects can occur when a reservoir is first formed, and submerged vegetation and soil decomposes. As it does so, it will deplete oxygen in the reservoir water. Deoxygenated water can be lethal to fish downstream.

Another water quality problem is mercury contamination. While mercury is often present in a harmless inorganic form in soil, once the soil is flooded, bacteria may transform this inorganic mercury into methylmercury, which is toxic, and can be absorbed, concentrated, and passed up the food chain. According to Canadian scientists, in every
case studied, the concentration of mercury in fish has increased from before the reservoir was constructed, to after.

4. Reduction of biodiversity due to blocking of movement of organisms and because of changes 1, 2 and 3 above.
Perhaps the most significant environmental consequence of dams is that they tend to fragment river ecosystems, isolating species populations living up and downstraem of the dam, and cutting off migrations and other movements. Of special importance is the blocking of migrating fish traveling up rivers, and then of smolt traveling back down rivers after they have hatched. In either case, the dam or reservoir can be an enormous obstacle, often with great impact on fish populations. For example, one immediate impact of settlement on Lake Ontario fish populations in the early to mid-19th century, was elimination of dozens of salmon runs in streams flowing into Lake Ontario, as these streams were blocked by milldams. This experience has been repeated dozens of times since then, on rivers all over the world.

Almost all dams also reduce normal flooding, effectively isolating the river from its floodplain, and eliminating the ecological benefits provided by this flooding.

The impacts of these changes are magnified by changes in the flow pattern of rivers downstream, that is caused by normal operation of dams. These changes, whether in total streamflow, in seasonal timing, or in short-term, even hourly fluctuations in flows, generate a range of impacts on rivers. This is because the life of rivers is usually tightly linked to the existing flow patterns of rivers. Any disruption of those flows, therefore, is likely to have substantial impacts.

For example, in Canada, rivers typically have their greatest flow in Spring, at snow melt, and their lowest flow during the winter. The function of dams, on the other hand, is to hold back this Spring flood, and release it during winter, when demand for hydropower is at its greatest. As a result, the Spring flood is greatly reduced, while winter flow may be greatly increased. Superimposed on this seasonal change are fluctuations in flows and levels, sometimes hour by hour, in response to changing daily demands for hydropower. The result is that existing rhythm of the river, in its ebb and flow, is disrupted, and along with it, all the habitats and species that depend on that rhythm.

Special topic: Dams and climate change
It has often been argued by supporters of dams that they provide a major environmental benefit, by reducing the need for other sources of electricity, such as coal- or oil-fired plants that generate greenhouse gases, and therefore contribute to the global climate change problem. However, in recent years there have been several reports that
hydroelectric reservoirs are actually themselves major sources of greenhouse gases, and so are not afterall a panacea for climate change. This has been a focus of active debate.

Sources:
o Fearnside, Philip M., "Hydroelectric Dams in the Brazilian Amazon as Sources of 'Greenhouse' Gases," Environmental Conservation, 1995, 22(1): 7-19.
o Fearnside, Philip M., "Hydroelectric dams in Brazilian Amazonia: response to Rosa, Schaeffer & dos Santos," Environmental Conservation, 1996, 23(2): 105-108.
o Rudd, John W. M., et al., "Are Hydroelectric Reservoirs Significant Sources of Greenhouse Gases?" Ambio, 1993, 22(4): 246-248. [A Canadian perspective.]
o Rosa, Luiz Pinguelli, Roberto Schaeffer, Marco Aurelio dos Santos, "Are hydroelectric dams in the Brazilian Amazon significant sources of 'greenhouse' gases?" Environmental Conservation, 1996, 23(1): 2-6. [Disputes Fearnside's claim of reservoirs as a significant source.]