Environmental impact of reservoirs
More recently, the construction of Three Gorges Dam and other similar projects throughout Asia, Africa and Latin America have generated considerable environmental and political debate.
The flushing flow method involves partially or completely emptying the reservoir behind a dam to erode the sediment stored on the bottom and transport it downstream.
[7] During experiments, it was found that suspended sediment concentration during flushing flows is double that of natural floods, although the total water discharge is lower.
For example, the eroded channel could create a lower water table level in the affected area, impacting bottomland crops such as alfalfa or corn, and resulting in a smaller supply.
[15] In the case of the Three Gorges Dam in China the changes described above now appears to have arrived at a new balance of erosion and sedimentation over a 10-year period in the lower reaches of the river.
[17] Once a dam is put in place represents an obstacle to the flux of nutrients such as carbon (C), nitrogen (N), phosphorus (P), and silicon (Si) on downstream river, floodplains and delta.
The stochiometric imbalance of nitrogen, phosphorus, and silicon of the outflow can have repercussion on downstream ecosystems by shifting the phytoplankton community at the base of the food web with consequences to the whole aquatic population.
[20][21][22] An example is the effect of the construction of the Aswan High dam in Egypt, where the drop in nutrient concentration to the Nile delta impeded the diatom blooms causing a substantial decrease the fish population of Sardinella aurita and Sardinella eba, while the reduced load of mud and silt affected the micro-benthic fauna leading to the decline of shrimp population.
[24] The operators of Burrendong Dam on the Macquarie River (eastern Australia) are attempting to address thermal suppression by hanging a geotextile curtain around the existing outlet tower to force the selective release of surface water.
[27] In many[quantify] low lying developing countries[example needed] the savanna and forest ecology adjacent to floodplains and river deltas are irrigated by wet season annual floods.
Dams generally discourage this cultivation and prevent annual flooding, creating a dryer downstream ecology while providing a constant water supply for irrigation.
However, MeHg concentration in lower food chain organisms remained high and showed no signs of returning to pre-flood levels.
[33] Resettlement Dams and the creation of reservoirs also require relocation of potentially large human populations if they are constructed close to residential areas.
"Dam related relocation affects society in three ways: an economic disaster, human trauma, and social catastrophe", states Dr. Michael Cernea of the World Bank and Dr. Thayer Scudder, a professor at the California Institute of Technology.
[34][page needed] At a dam in Brazil, where the flooded basin is wide and the biomass volume is high the methane produced results in a pollution potential 3.5 times more than an oil-fired power plant would be.
[35] A theoretical study has indicated that globally hydroelectric reservoirs may emit 104 million metric tonnes of methane gas annually.
This is not an isolated case, and it appears that especially hydroelectric dams constructed in lowland rainforest areas (where inundation of a part of the forest is necessary) produce large amounts of methane.
"But even including methane emissions, total GHG [Green-House Gas] per KWh generated from hydropower is still at least half that from the least polluting thermal alternatives.Thus, from the perspective of global warming mitigation, dams are the most attractive alternative to fossil fuel based energy sources.
