Lakes and reservoirs contain about two-thirds of Earth’s surface and atmospheric freshwater. Managing water in these waterbodies can help communities mitigate seasonal and even annual variability in local precipitation and water runoff.
Reservoirs produce a relatively dependable water source that can be managed with some confidence. When water demand spikes, perhaps during spring’s agricultural boom, reservoirs can meet excess demand with resources that have been collected during the entire year.
Artificially-dammed reservoirs can also provide hydroelectric power, often on a scale that makes them attractive to many policy-makers. At the end of the twentieth century, there were over 45,000 dams throughout the world with a height of 15 meters or more. Globally, an estimated 160 to 320 new large dams are being built every year.
But reservoirs do have their drawbacks. Open water is subject to evaporation at rates that vary according to climate. In some areas, 20 percent of the total yearly runoff is lost to the atmosphere.
Reservoirs may cause significant environmental damage by preventing the movement of sediment and thereby creating erosion downstream, by interrupting the life cycle of fish species that must travel upstream to spawn, and by changing the timing and amount of flow downstream, which can have impacts on plant and animal species in the river and its floodplain.
The social disruption caused by dam construction is significant for those communities that must be resettled because their property—often highly fertile agricultural land—will be submerged. The World Commission on Dams estimates that worldwide 40 to 80 million people may have been physically displaced in this manner. Displacement has direct economic costs by separating people from their means of livelihood (fishing, farming, and ranching, for example). It also has social costs due to the disruption and transformation of communities and their traditional social structures.
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.