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THREATS TO TROPICAL RIVERS AND LAKES

By Rhett Butler   |  Last updated July 31, 2012
Tropical rainforest waters are highly threatened today by hydroelectric projects, erosion from deforestation, overfishing, and pollution from industrial activities, including oil spills and mining waste. The effects from the degradation of these waters are widespread, inflicting damage on the global economy, the environment, and local peoples.

Dams


Balbina dam outside Manaus, Brazil
The Balbina dam flooded some 2,400 square kilometers (920 square miles) of rainforest when it was completed. Phillip Fearnside, a leading expert on the Amazon, calculated that in the first three years of its existence, the Balbina Reservoir emitted 23.75 million tons of carbon dioxide and 140,000 tons of methane, both potent greenhouse gases which contribute to global climate change.
Increasing demands for energy are putting the world's rivers at risk, with hundreds of dams planned in the Amazon Basin, Borneo, the Mekong watershed, Central America, and Central Africa. Hydroelectric projects have been responsible for flooding vast areas of rainforest, with significant detrimental impacts for climate, wildlife, and river-dependent people.

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 what Philip Fearnside, an expert on the Amazon, calls a "methane factory".

Dams in the Amazon.
Planned dams in Brazil. Courtesy of Dams in the Amazon.


Dams can also be enablers of deforestation, spawning roads that facilitate new clearing and generating electricity for industrial farms, mines, and aluminum manufacturing. For example, the controversial Belo Monte dam on the Xingu River in the state of Pará, Brazil, illustrates many of the concerns with dam-building in the Amazon, where 146 dams are planned in coming decades. The dam will flood a quarter of the city of Altamira and divert 80 percent of the Xingu's flow from the main stem of the river, leaving 60 miles (100 km) of one of the Amazon's largest tributaries nearly dry. Communities of that stretch of the river will lose their primary source of livelihoods — fishing. The dam will also impede migration of some of the river's largest fish species, potentially affecting fish populations elsewhere in the river basin. (A number of Amazon fish species move upriver or downriver to spawn). Other communities will suffer from inundation. In these areas, people are being removed by force, according to Fearnside.

But the dam has bigger problems. As currently designed, Belo Monte will suffer from the Xingu's seasonal variation in water flows. The most expensive parts of the dam to operate — turbines and transmission lines — would need to be idled for four months each year due to low water. That issue makes it likely that Brazil will push forth with a plan to build additional dams upstream from Belo Monte (up to six dams were planned on the Xingu until 2008). These dams would capture water, ensuring more consistent flow for Belo Monte. However greater water level fluctuation upstream means these dams will have even higher methane emissions relative to their generating capacity.

Damming the Amazon



Erosion


Erosion is a conspicuous impact of deforestation with serious consequences for river commerce and river life. Sediments build up creating sandbars and shallows and interfering with river transportation. Similarly, sediment build-up reduces the effectiveness of existing hydroelectric projects. Erosion and the resulting decline in water clarity can cause downstream mayhem for offshore coral reefs. River inhabitants also suffer due to the reduced water clarity. Species that rely primarily on sight decline the most, while the increased amount of suspended particles interferes with fish gills. Erosion inhibits plant growth and can hinder the development of fish eggs.

Overfishing


Overfishing is a problem that plagues the world's oceans (35-60 percent are overfished worldwide [overfishing news]) and freshwater habitats. Regional declines in catch have been reported throughout the Amazon. The loss of certain species responsible for seed dispersal will have a negative effect on the renewal of the rainforest.

Pollution


Poisoning from spills and pollution from industrial processes mining, and sewage impact the diversity of rainforest waters, in addition to affecting human populations. Chemical spills are usually associated with oil development and mining.


Turucui dam in Brazil
Satellite image of the Turucui dam and associated deforestation in Brazil. (Photo courtesy of DigitalEarth)



Review questions:

  • How are rainforest rivers under threat?
  • How do dams contribute to global warming?
  • Why are dams usually bad for native fish species?

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Continued: People of the rainforest

  • Oil operations in the Niger River Delta are examined in Moffat, D. and Lindén, O., "Perception and Reality: Assessing Priorities for Sustainable Development in the Niger River Delta," Ambio Vol. 24 No. 7-8 (527-538).
  • Pakenham, T. (The Scramble for Africa, New York: Avon Books, 1991) provides the history of the European discovery of the Niger River.