Our planet Earth is extraordinarily rich in natural resources - except for water. In most countries, however, water is a scarce resource. Increasing demand for water, and decreasing water quality, make careful water management a priority in each country. In many countries large scale engineering has been used to store water behind dam walls, and to distribute water from regions of plenty to regions of need. Normally a great part of the annual rainfall is stored in dams.
Dams have both positive and negative ecological impacts.
They can be beneficial in that they regulate the flow of a river, reducing flood damage, and contributing to a perrenial rather than seasonal flow. In addition, sediment is deposited in a dam, and the growth of aquatic plants means that excess nutrients are removed from the water. Thus water leaving a dam might be cleaner than water entering it. Negative ecological impacts include a reduction in strong water flow which reduces the river's scouring ability. This, in turn, can lead to silting of estuaries.