During last few centuries the environment has been impacted by anthropogenic activities with increasing speed. The impact has caused loss of natural character and function of many components of the environment. The tendency of changing natural systems is obvious all around the world and the pressure is exponentially increasing with growing human population. The impact on a natural ecosystem is demonstrated by the ecosystem degradation and decreasing ability to recover. At present, numbers of ecosystems are so heavily affected that they are not able, without human action, to be recovered and to accomplish natural function. The negative impact of human activities on the environment slows down during last few decades due to accepted legislative measures and the degradation of the environment has been minimized. One of the main legislative tools to minimize the anthropogenic impact on the environment is environmental impact assessment (EIA) (in some sources 'environmental impact analyses').

EIA evolved during the 1970s and 1980s in developed countries, in response to the need to address potential impacts of proposed development early in the development planning, rather than at the final design stage. Experience indicated that project completion targets and financial imperatives in place by the final design stage, largely preclude the consideration of alternative development arrangements or process securing better environmental outcomes.

activity compared with the situation which would have occurred had the activity not been initiated. Environmental systems are not static, but change over the course of time even without the influence of man. Some are very dynamic, while others only change imperceptibly. In order to make predictions about impacts, assumptions have to be made about natural changes. A major deficiency of many studies has been failure to establish a time frame indicating when impacts are likely to be manifest. Impacts are also site-specific and hence determination of their spatial distribution is also important. Spatial aspects are usually considered more adequately than temporal ones. It is useful to distinguish between direct (primary) and indirect (secondary, tertiary, and higher-order) impacts. Some impacts are a direct consequence of a particular activity. For example, without adequate mitigating measures, construction of a dam on a river will prevent the upward movement of migratory fish. This would be a direct impact of the project. Other impacts, however, occur as a result of changes in a chain of environmental parameters. Thus, to continue this example, there would also be indirect impacts upon fish population. Reductions in stream flow and turbulence would lower the oxygen tension and affect survival. Reduced water flow would also affect the nature of the streambed, the consequent siltation-making conditions unsuitable for migratory fish to breed, the changes in habitat nature will change the possibility for fishes to hide, feed, rest, and consequently the diversity of the aquatic community will change.

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