About two-thirds of the Earth’s surface and atmospheric fresh water is contained in lakes and reservoirs. Water management in these bodies of water can assist communities in reducing seasonal and even annual variations in local precipitation and runoff
Reservoirs provide steady water supply that can be managed with certainty. Reservoirs can meet demand with resources acquired over the course of the year when water demand increases, such as during the spring agricultural boom.
Hydro-electric power is commonly generated from dammed reservoirs on a large enough scale to appeal to many policymakers. At the end of the twentieth century, there were about 45,000 dams with a height of 15 meters or greater around the world. Every year, an estimated 160 to 320 new big dams are developed around the world.
Reservoirs, however, have some limitations. Open water evaporates at different rates depending on the climate. In some locations, up to 20% of total annual runoff is lost in the process of evaporation.
Reservoirs can harm the environment by preventing sediment from moving downstream, causing erosion, disrupting the life cycle of fish species that must migrate upstream to spawn, and altering the timing and amount of flow downstream, all of which can affect plant and animal species in the river and its floodplain.
Dam construction causes severe social disturbance for people that are forced to relocate because their property-often exceptionally productive agricultural land-will be submerged. According to the World Commission on Dams, 40 to 80 million people may have been physically relocated as a result of these dams. Displacement has immediate financial implications since it separates people from their sources of income. Due to the disruption and restructuring of communities and their old social systems, it also bears social costs.
Finally, dammed structures are subject to sedimentation, which over time diminishes their capacity to store water. When runoff water is trapped behind dams, there is no outlet for the sediments that are carried into the reservoir with running water. Sometimes mud can be drained from these reservoirs, but the process requires large volumes of water to “flush” mud downstream—where it can have negative environmental impacts.