A boom in construction of major hydroelectric dam projects on the Amazon, Congo and Mekong rivers increasingly threatens a range of rare and unique freshwater biodiversity according to a new study published inScience.
Existing dams on the three basins are generally small and located in upland tributaries, but over 450 additional major dams are planned, with some already under construction. Most of these dams are planned to be built in areas of fast water flow – such as waterfalls and rapids – which are often hotspots of high biodiversity.
These three river basins hold roughly one-third of the world’s freshwater fish species. The 450 additional dams being planned or under construction in these basins put many unique fishes at risk.”
Lead author Kirk Winemiller, professor of wildlife and fisheries sciences at Texas A&M University
The challenges of dam construction in areas of unique freshwater biodiversity
The authors of the study, a team of scientists from 30 academic, government, and conservation organisations in eight countries, suggest that proposals for major hydropower projects in the three basins regularly overestimate the economic benefits of their construction whilst underestimating their environmental impacts.
The authors recommend that improved approaches to dam planning, siting and evaluation are crucial. Strategic basin-scale planning that balances the potential benefits of hydropower production with the need to sustain biodiversity and ecosystem services is needed. Such trade-off analyses are now possible due to the development of increasingly comprehensive biodiversity, socioeconomic and energy datasets for the basin areas.
Without such careful and large-scale planning, the study argues that increasing dam construction on the Amazon, Congo and Mekong basins has the potential to significantly reduce rare and endemic freshwater biodiversity, and to compromise the livelihoods of the human communities that depend on the river ecosystems.
Environmental impacts of dams at different scales
Recent scientific research suggests that dam site selection strongly influences the environmental impacts of construction. Dams inevitably impact freshwater biodiversity at a local scale, for example, by changing water quality and hydrology. Dam construction can catalyse a phenomenon known as an ecological regime shift, where a dynamic and complex ecosystem becomes more homogeneous and less productive. Studies of existing tropical reservoirs created above dams (pdf) have found they are often dominated by a small number of common fish species and often inhabited by non-native species introduced for angling or aquaculture.
Dams also have much wider environmental impacts, significantly in blocking migration routes and fragmenting fish and animal populations, particularly for species that require different habitats (e.g. flood plain nursery areas) at different stages of their life-cycles. Research conducted in Brazil on fish passages (or ladders) suggests that they are largely ineffective (or even damaging) in facilitating the movement of migratory fish stocks. The effects of dam construction may also be seen in river estuaries and deltas, and even in the marine ecosystems they feed, as changes to upstream nutrient and sediment dynamics cascade downriver.
So, with the knowledge of this range of environmental effects at different scales, the authors recommend that to minimise biodiversity loss in tropical river basins, planning for any proposed dam construction must take place at the basin scale.
Planning for the ecological, economic and social impacts of dam construction
The authors suggest that planning and approval processes for large hydropower dams are rarely comprehensive or transparent and regularly overestimate the economic benefits of dam construction. For example, the The Inga I and II dams on the Congo, constructed in the 1970s and 1980s, currently yield only 40% of the 2132-megawatt installed capacity.
Planned additional dams – Inga III and Grand Inga – would harness as much as 83% of the river’s annual discharge, significantly diverting and reducing water flows downstream. It is also suggested that hydropower dam proposals often underestimate the costs of mitigating the environmental damage they cause. For example,around $26 billion has been spent so far on mitigating the environmental impacts of the huge Three Gorges Dam in China.
Long-term ripple effects on ecosystem services and biodiversity are rarely weighed appropriately during dam planning in the tropics. There is good reason for skepticism that rural communities in the Amazon, Congo, and Mekong basins will experience benefits of energy supply and job creation that exceed costs of lost fisheries, agriculture, and property. An improved approach to dam evaluation and siting is imperative.
Co-author Peter McIntyre, assistant professor of zoology in the Center for Limnology at the University of Wisconsin–Madison
Freshwater biodiversity isn’t evenly distributed throughout these huge river basins, and many sub-basins and tributaries contain unique species that aren’t found elsewhere. Lead author Kirk Winemiller explains, “The Xingu River, a major Amazon tributary, provides a good example of this. The lower stretch of the Xingu is a complex of rapids that provides habitat for about four dozen fish species found nowhere else on Earth.”
These endemic fish species living in the Xingu are now threatened by the Belo Monte hydroelectric project, which Winemiller argues will “radically change the river, its ecology, and the lives of local people, especially indigenous communities that have depended on the river’s ecosystem services.” Indeed, it has been suggested that the Belo Monte dam may set a new record for biodiversity loss as a result of construction due to the selection of a site with exceptional species endemism.
The impacts of the hundreds of proposed Amazon dams are also likely to include forced relocation of human populations and expanding deforestation. Co-author Leandro Castello, assistant professor of fish conservation at Virginia Tech explains:
Even when environmental impact assessments are mandated, millions of dollars may be spent on studies that have no actual influence on design parameters, sometimes because they are completed after construction is underway. A lack of transparency during dam approval raises doubts about whether funders and the public are aware of the risks and impacts on millions of people.
Recommendations for balancing major dam construction and freshwater biodiversity conservation
Because of their immense biodiversity and critically important fisheries and other ecosystem services such as floodplain agriculture, tropical rivers pose a special challenge for hydropower development. We are advocating for improved assessment of hydropower costs and benefits based on more comprehensive, science-based, and timely evaluation of hydropower potential, biodiversity, ecosystem services, and socioeconomic patterns at the river basin scale.
The study suggests that for the first time, spatial data on biodiversity and ecosystem services in the three basins can support new analyses that balance the costs and benefits of hydropower construction.
New analytical methods can be used to study the cumulative impacts of multiple dams on interacting factors such as hydrology, sediment dynamics, ecosystem productivity, biodiversity, fisheries, and rural livelihoods throughout watersheds. “Incorporating these data and tools into assessment protocols would boost the credibility of dam siting in the eyes of all stakeholders,” argues Winemiller.
The study concludes with a set of firm recommendations:
Institutions that permit and finance hydropower development should require basin-scale analyses that account for cumulative impacts and climate change. Common-sense adjustments to assessment procedures would ensure that societal objectives for energy production are met while avoiding the most environmentally damaging projects.
Without such careful planning, the authors suggest that the proposed dam construction on the Amazon, Congo and Mekong basins has the potential to significantly impact the rich biodiversity and diverse ecosystem services that they support.