Concerns over undesirable environmental and social effects have been the principal barriers to hydro worldwide. Because most hydroelectric projects depend on dams, a river habitat is often replaced by a reservoir. Conditions for wildlife and aquatic creatures can be radically altered. Proper sitting, design, and operation can mitigate many of these problems, but more difficult challenges arise when human populations are forced to relocate. In some developing countries, the economic well-being and health of affected populations have declined after relocation.
Protection of fisheries is often one of the most contentious environmental issues with hydropower development. Most countries require that a minimum flow be maintained in the river to ensure the life and reproduction of indigenous fish and the free passage of migratory fish. The determination of an acceptable minimum flow is a key issue in the economic viability of any hydro scheme. To date, there is no universally accepted method of determining this flow to the satisfaction of both developers and regulators.
The construction of hydropower systems can have temporary effects on the local environment, particularly on water quality, such as muddying the water downstream from the development. Temporary access for construction vehicles can also cause disturbance, though once established, run-of-river hydro schemes have minimal visual impact. The principal permanent impacts are on the depleted stretch of the watercourse, where mitigating measures need to be taken to sustain river ecology and fisheries. Some types of turbines provide increased oxygenation of the tailrace water, thus improving water quality.
In the last few years, more emphasis has been put on the environmental integration of small hydro plants into river systems in order to minimize environmental damage. The requisite technology can be considered commercially and technically mature, although improvements are possible to make it suitable for export to rapidly expanding non-OECD markets. Innovations in civil engineering design, electromechanical equipment and control are possible, as well as instrumentation and systems which mitigate environmental effects.
Other challenges for small hydro include regulatory delays for sitting and permitting, as well as the burden of lengthy environmental impact reviews and assessments, which are often as rigorous as for large hydropower projects.
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