In the last half century, dams have been built on most of the world's great rivers. Designed to provide power, irrigation, drinking water and flood control they have also attracted controversy and protest. The looming threat of climate change is now revealing new dam-related problems.

Devastating droughts are ravaging huge areas of Asia and Africa, and scenes of some of the worst flooding in human memory have been hitting the headlines with alarming regularity. The UN's Intergovernmental Panel on Climate Change estimates that around the world, 170 million more people will face extreme water shortages as a result of global warming.

This week, Earth Report visits two climate change hotspots in India and Southern Africa, where the tell-tale signs of a changing hydrological system are beginning to show. Will current methods of managing water be able to cope under ever more extreme weather patterns?
 

The Problem with Dams

Dams are built for the purpose of capturing water in order to generate hydroelectricity and to provide water for cities and irrigation projects. Dams are built with the promise of reducing flooding downstream by capturing the flood flows of rivers, and this usually works well dueing moderate flows. But in times of heavy rains and exceptional river flows, the dams' ability to capture water actually becomes a menace. As the reservoirs fill up, the choice is between the dam overfilling, leading to massive floods, or to make emergency releases of water. As the latter will also cause some flooding it is not a popular decision to make, with the result that it is often taken far too late. The result is emergency releases that are far greater and more sudden than flows that would occur during natural river flooding.

These dam-caused floods are especially destructive because they are unexpected. People live on the floodplains downstream of dams in the belief that they are safe.
 

Changing the Climate

The construction of large dams can change the natural environment to the extent that it affects the hydrological systems in a particular area, thereby making river movements less predictable and dams less able to prevent floods. The deforestation of river catchments or the draining of surrounding wetlands means there are fewer natural "sponges" for floodwaters increases the risk of extreme floods. Moreover, climate change affects rainfall patterns and timing and makes rainfall more erratic. This is increasingly causing flood disasters in many parts of the world:
 

Meteorological Mayhem: Orissa

For more than a decade, the state of Orissa in India has been reeling under extreme weather conditions. Heat-waves, cyclones, droughts and floods have earnt it a reputation as the disaster centre of India. Its location at the head of the Bay of Bengal makes Orissa especially vulnerable to cyclones which build up in the Indian Ocean. But the cyclones are not the only problem. The climate as as a whole has become increasingly unpredictable.

Indian scientists believe that Orissa's extreme floods and increasing droughts could be a dress rehearsal for the meteorological mayhem facing many parts of the globe. The natural order of the seasons seems to have been thrown out of balance. Instead of the traditional six seasons in Orissa, only three remain. As summers become longer, farmers find that their ancient methods of forecasting the weather are failing: "Our local predictions of rainfall has been failing, normally we could tell when rain was coming by the different behaviours of the birds, If a small bird called Tingchalchal danced from one bush to another we knew that rain was coming, but now it dances but there is no rain."
 

Caught in the Cycle

Caught in the relentless cycle of droughts and floods, the poorest of the poor have become even more vulnerable. In the village of KundaButla in Orissa's Bolangir district, consecutive years of drought and flash-floods have caused desperation. Last year,  Lalita and Shiamlal Tandi were faced with four sick children and no food or income. They had to make the agonising decision to sell one of their daughters to the landlord in order to save her and the rest of the family. Many destitute people faced with water and food shortages are forced to borrow money to migrate to neighbouring states where they work as labourers to repay their debts. Critics argue that the government could help reduce the miseries of migration by improving disaster mitigation strategies and relief measures.
 

Choking with Silt

The paradox in western Orissa is that the area is suffering from drought despite receiving an average overall volume of rainfall. But the rainfall is erratic and unpredictable. While climate change may be making life in Orissa more difficult, there is also a growing understanding that development schemes based on western technology are actually making the problems worse.

In the past villages would catch and store rainwater in traditional pond like structures called Kata's. But after independence in 1947, the traditional structures were neglected as the government's focus turned to large-scale irrigation schemes. The Hirakud Dam and canal projects were built for large-scale irrigation downstream.

But today there are huge problems. While the areas below the dam are now waterlogged and suffering from excess salt in the soil, the entire catchment area above the dam has been deforested, and the huge 900 square km reservoir has become choked up with silt as a result. Most of the Kata's have silted up and are no longer able to store water from large downpours.

Today it's those villages that protected their traditional water storing methods, and their forests, which stand out as those best equipped to withstand the consecutive calamities.

There is growing consensus that the government must take measures to help people adapt to a variable climate,  and that reforestation and water conservation must be a central strategy. People on the ground are hoping that their indigenous knowledge will be taken seriously at the Third World Water Forum in Japan in March 2003.
 

The Mighty Zambezi

Over the millennia, life in central Mozambique was measured by the ebb and flow of the great Zambezi River. Every year the wet season waters would spill over into the Zambezi's vast floodplains, nourishing the soils for millions of people and countless species of wildlife. In the dry season the low waters would recede, allowing people to move back onto the floodplain and harvest the riches created by the Zambezi waters.

But in the last 40 years two large dams have stopped the Zambezi's seasonal rhythm. Kariba Dam in Zambia and Zimbabwe and Cahora Bassa Dam in Mozambique now control the Zambezi's waters, storing them in vast reservoirs to generate hydropower. With the dams controlling the floods there is no reason for the annual migration off the floodplain so people have settled permanently along the riversides.

But by interrupting the natural flow of the river, the dams have created a false sense of security. When for example rising floodwaters occur, the dam has to be opened so that it doesn't overflow. But the people now living permanently in the low-lying areas along the Zambezi are not prepared for the rise in water. When the villages flood, those who aren't killed are  forced to flee.
 

Drying out the Wetland

To minimize the risk of major flood disasters, the dam managers release pulses of water during the dry season to make space in the reservoir for runoff from the hills above the dam. But these unexpected releases wash out the subsistence crops of farmers along the river.

The fishermen of the Zambezi also have a sorry tale to tell. Their once abundant fisheries have shrunk to pathetic proportions since the Cahora Bassa dam was built, and high dry season flows make what fish are left in the river harder to catch. According to their explanation, the dam stops the fish from swimming downstream.

According to the scientists the real reason for the shrinking catch is the loss of regular rainy season floods. This has caused an overall drying out of the floodplain delta so that the fish can no longer spill over into their spawning grounds to breed. The entire wetland ecosystem has been all but destroyed.
 

Adapting to the Threat of Climate Change

The solution being proposed is to restore the natural flooding patterns of the river in an effort to save what was once one of the most productive and diverse wetland ecosystems in Africa. Scientists argue that annual floods would not only benefit hundreds of thousands of people along the lower Zambezi, but would restore critical habitats for many endangered species in the delta, and save the dying coastal mangroves. People on the ground support the idea of the restoring natural seasonal floods to the river at the traditional time of year.

If danger signals like the flooding in Orissa and in Mozambique are ignored, the poor, as always, will be the first to lose out. But it is politicians who must ultimately be convinced about the need to prepare for future cycles of droughts and floods. Scientists and water managers gearing up for the Third World Water Forum are hoping to persuade ministers that strong and appropriate commitments are needed to adapt to the threat of climate change.