Changes

* Evaporation of the water in the reservoirs upstream from the dams becomes a problem in some cases, particularly in arid regions where fresh water is a priceless commodity, needed for agriculture and human consumption.

* Evaporation of the water in the reservoirs upstream from the dams becomes a problem in some cases, particularly in arid regions where fresh water is a priceless commodity, needed for agriculture and human consumption.

* Hydroelectric plants are not necessarily inexhaustible sources of electricity. They have a limited operating life. The reservoirs behind the dams become silted up in many cases with the consequent long-term loss of water storage capacity. The sediment caught at the dam can sometimes cause the erosion of the turbines if it reaches the power intakes. The retention of sediment behind the dam can also be the cause of serious problems downstream, where the reduced influx of sediment often results in important changes of the river valley, sometimes all the way to its [[Deltaic environments|delta]]. The Aswan High Dam in Egypt and the Hoover Dam in southwestern United States are good examples of such problems. In Egypt, for thousands of years, farmers along the Nile Valley counted on the yearly floods to bring organically rich silt that fertilized their intensely cultivated plots. Now, the silt piles up behind the Aswan Dam.

* Hydroelectric plants are not necessarily inexhaustible sources of electricity. They have a limited operating life. The reservoirs behind the dams become silted up in many cases with the consequent long-term loss of water storage capacity. The sediment caught at the dam can sometimes cause the erosion of the turbines if it reaches the power intakes. The retention of sediment behind the dam can also be the cause of serious problems downstream, where the reduced influx of sediment often results in important changes of the river valley, sometimes all the way to its [[Deltaic environments|delta]]. The Aswan High Dam in Egypt and the Hoover Dam in southwestern United States are good examples of such problems. In Egypt, for thousands of years, farmers along the Nile Valley counted on the yearly floods to bring organically rich silt that fertilized their intensely cultivated plots. Now, the silt piles up behind the Aswan Dam.

* Finally, the development of large-scale hydroelectric power requires sites that allow the building of dams that provide a high head (height of water fall), located in a river with high water flow, with large water-storage capacity, and, preferably, near centers of electricity consumption. These dams are sometimes called ldquolarge damsrdquo and are defined as having a height of 15 m (50 ft) or more from the foundation or, if only 5–15 m (16–50 ft) in height, having a reservoir volume of more than 3 million m³ (177 million ft³). Sites favorable for the building of large dams are limited in number and unevenly distributed throughout the world. They are not found, for instance, in arid regions with low rainfall and, therefore, with no permanent streams, or in regions of low relief. Although most of the world's sites for the building of hydroelectric power plants have been identified, only a relatively small proportion have been developed, mostly in the industrialized countries. At least seven European countries (Austria, France, Italy, Norway, Spain, Sweden, and Switzerland) generate substantial percentages of their electricity in hydroelectric plants.

* Finally, the development of large-scale hydroelectric power requires sites that allow the building of dams that provide a high head (height of water fall), located in a river with high water flow, with large water-storage capacity, and, preferably, near centers of electricity consumption. These dams are sometimes called "large dams" and are defined as having a height of 15 m (50 ft) or more from the foundation or, if only 5–15 m (16–50 ft) in height, having a reservoir volume of more than 3 million m³ (177 million ft³). Sites favorable for the building of large dams are limited in number and unevenly distributed throughout the world. They are not found, for instance, in arid regions with low rainfall and, therefore, with no permanent streams, or in regions of low relief. Although most of the world's sites for the building of hydroelectric power plants have been identified, only a relatively small proportion have been developed, mostly in the industrialized countries. At least seven European countries (Austria, France, Italy, Norway, Spain, Sweden, and Switzerland) generate substantial percentages of their electricity in hydroelectric plants.

The building of large dams requires large initial capital investment that is not always rewarded with favorable economic profitability. The reported returns on the investments made in hydroelectric power plants have increasingly been questioned.

The building of large dams requires large initial capital investment that is not always rewarded with favorable economic profitability. The reported returns on the investments made in hydroelectric power plants have increasingly been questioned.