Imperiled Riches—Threatened Rainforests


July 22, 2012

[More on hydroelectric projects] Large hydroelectric projects — traditionally funded by international aid and development organizations like the World Bank, but increasingly funded by national development agencies and banks like Brazil's BNDES and state-backed banks in China — have led to widespread forest loss. Besides inundating large tracts of rainforest (dams in the lowland areas like much Amazon are generally ecologically inefficient because large tracts of forest are flooded due to the flatness of the basin) and killing off local wildlife, the dams have the effect of destroying aquatic habitats and affecting fish populations, displacing Indigenous peoples, and adding carbon to the atmosphere.

On top of the ecological damage, several projects have silted up from the erosion resulting from deforestation, rendering the dams inefficient. The reduced water flow downstream disturbs riverbeds and affects floodplain farmers who rely on seasonal floods for nutrients to enrich the soil and kill pests. Thus they may turn to pesticides and artificial fertilizers which have their own negative environmental effects. Diminished water flow can contribute to greater influx of salt water in river deltas, affecting coastal ecosystems essential to fisheries. Hydroelectric projects are also of concern from a health standpoint because they provide opportunities for the spread of disease-carrying organisms including snails (schistosomiasis/bilharzia) and mosquitoes (dengue fever, yellow fever, malaria).

Greenhouse gas emissions from dams

Dams are often touted as sources of "green" energy, but in the tropics, recent research suggests this is a misnomer. Dams in the tropics have two principle greenhouse gas emissions sources: carbon released from soil carbon stocks and dying vegetation when the reservoir is flooded and methane formed where organic matter decays under low oxygen conditions at the bottom of the reservoir. Methane emissions are facilitated by a dam's turbines, which usually draw from the bottom of the reservoir and spray methane-dense water into the air upon release. Emissions from rotting vegetation occur on an ongoing basis when the levels of the reservoir fluctuate: during the dry season weeds, emerge from the muddy drop-down zone, only to rot again when waters return. The effect turns a typical tropical dam into a "methane factory", as coined by Philip Fearnside, a scientist who has published widely on the issue.

An indirect source of emissions from dams is the activity their electricity powers. For example, several major dams in the Amazon are under construction specifically to power mining operations and agroindustry, both of which can drive deforestation. The case is similar in the Malaysian state of Sarawak on the island of Borneo, where hydropower will fuel new mines, ore refiners, and mills for palm oil and paper production.

Dam expansion

Despite vocal objections to many dams from civil society and increased awareness among traditional lenders like the World Bank, the number of planned dam projects in tropical regions is growing. On the Mekong alone, one of tropical Asia's biologically richest rivers, some 11 dams are planned by 2030, while 77 hydroelectric projects are in the works for the Mekong Basin. Meanwhile some 150 dams are planned in the Amazon Basin.

Lake Balbina, a man-made reservoir created to supply hydroelectric power to the city of Manaus in Brazil. Photo courtesy of Jacques Descloitres, MODIS Rapid Response Team, NASA/GSFC.

Review questions:

  • What environmental problems can dams cause?

Other versions of this page

spanish | french | portuguese | chinese | japanese

Continued / Next: Pollution