Human Impacts - Impoundments and Water Chemistry--
Dams or impoundments create reservoirs. The reservoirs are used for domestic water supply, irrigation and industry, and energy production. However, building and operating dams modifies the aquatic ecosystem. The effects occur most prominently locally and receive considerable attention on regional levels. However, the alteration due to the establishment of large dams causes effects far beyond the local scale, transgressing national boundaries, and reaching far outside the system including deltas and estuaries. Dams alter flow, temperature and sediment regimes of rivers impacting not just the biota but the physical system as well. The effects of impoundments are felt differently between the upstream and downstream sections of a river.

Upstream effects. When a barrier is put across a river, the ecological conditions change dramatically for some distance upstream as the aquatic system essentially switches from a river to a lake. Deposition of fine sediments on the original river substrate can result from the reduction of current behind the dam. The decomposition of drowned vegetation by bacteria releases nutrients but also decreases oxygen levels, possibly leading to deoxygenation of bottom waters. Oxygen depletion can trigger the reduction of nitrate, manganese (hydr)oxides, iron (hydr)oxides and sulfate into components such as Mn(II), Fe(II), NH₄⁺, and H₂S which can accumulate in deep waters. However, while being toxic to aquatic organisms, these reduced compounds can also enhance the productivity of a stream. The presence of these ions in solution can keep the sediment water interface anoxic by preventing the build-up of iron hydroxide layers and thus reduce the capacity of sediments to retain phosphorus.

Downstream effects. Downstream of the dams, the river tends to lose much of its dynamic nature as flow patterns are mediated and more regular, although they can also become more extreme. A number of other physical changes take place.

• Flow patterns. Where water is withdrawn from a reservoir (e.g., for agricultural purposes or through evaporation), then clearly the overall river discharge downstream of the dam will be reduced. Flow regimes below the dams are also altered, but the new regime depends on the use of the reservoir.

• Temperature. The location of reservoirs on river channels can significantly alter temperature regimes. In the summer if the outflow is from surface waters of the reservoir, the downstream system can be warmer than in unregulated rivers and can often have high oxygen levels.

• Water Quality. Since the oxygen concentration in the discharged water corresponds to the reservoir concentration at withdrawal level, deep outlets may release low-oxygen water into downstream rivers during periods of stratification. Such water may also contain reduced compounds (see earlier discussion on upstream effects) that may lead to reduced assimilation capacities of the downstream river.

• Sediment loads. Much of the sediment load carried by a river can be deposited behind the impoundment and downstream levels can be significantly reduced. Not only is the amount of sediment altered, but the nature of the suspended organic matter also changes from largely detritus to plankton which can persist for long distances in large rivers.

• Morphology. Impounded streams can change from cold, turbid, and/or torrential braided channels to narrower, single thread meandering channels below dams. In many parts of the world, there is a diverse and broad riparian zone that is maintained by annual flood disturbances. Under strong river regulation, this zone shrinks and the surrounding forest expands towards the river margins.

The downstream effects are not just confined to the river system. The regulation of water decreases or eliminates regular inundation and deposition of nutrient-rich sediments on the flood plain, breaking the lateral links between channel and land in the lower reaches of the river. This can have profound effects on both aquatic and terrestrial ecosystems.

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