When massive flooding in Haiti and the Dominican Republic killed 3000 people and left tens of thousands homeless late last month, everyone seemed sure who was to blame.

"Haiti's deforestation allows flood water to run unchecked," declared USA Today. Haiti's prime minister pointed the finger at poor farmers for cutting down trees for fuel and to make charcoal. The Associated Press ran touching interviews with the elders of the flood-ravaged Haitian town of Mapou about how they had been forced to fell trees to cook their food even though they knew it would eventually bring about their own destruction. France's foreign minister promised aid to reforest the denuded hillsides.

It was a predictable response. Just about every time there is a major flood anywhere in the world, small farmers and loggers are held to account. Floods in Bangladesh are blamed on forest clearing in the Himalayas. In the late 1990s, loggers took the rap for the thousands who died, and the billions of dollars of damage done, during Hurricane Mitch in Central America, and for the floods along China's Yangtze river. Indeed, the idea that loggers and small farmers help cause devastating floods is so ingrained inmost people's minds that few would think to question it.

But the idea is deeply flawed. There is not a shred of scientific evidence to suggest that logging or deforestation play significant roles in massive floods. And the myth is doing great damage to farmers who need forests to survive.

One reason that politicians, newspaper editors and even conservationists continually get it wrong is because deforestation and logging clearly contribute to more localised floods. If you remove trees, less water evaporates into the atmosphere and flows into the soil, so more of it runs off into streams and rivers. Unprotected soil gets washed into rivers, where it builds up, making them shallower and more likely to overflow. Trees also protect against mudslides by stabilising the soil. Numerous experiments have confirmed all this.

But these experiments focus only on small areas and on moderate floods that occur relatively frequently. None of them has analysed entire regions or the once-in-a-lifetime floods that do the most damage. Whenever scientists have tried to find links between forests and larger-scale, unusually severe flooding, they have consistently drawn a blank.

At the large scale, things like dams and drainage channels and how much water people consume are far more significant Usually rainfall is fairly localised, but big floods tend to occur on those relatively rare occasions when it rains everywhere at the same time. In this case, the soils get completely soaked early on. Once that happens they cannot absorb any more water, so whether there are forests or not the water all runs off into the streams. And while trees may prevent small mudslides, they have little effect on huge landslides.

Given our limited knowledge, we still cannot completely rule out the possibility that removing forests might make major floods marginally worse - perhaps by increasing silt build-up in rivers, for example. However, if such an effect does exist, it is most probably rather small. If you want to limit the damage from large floods, flood control measures such as drainage channels and keeping people out of risky areas are more effective methods than trying to stop people cutting down trees.

You might ask whether it matters that people get it wrong. Why not let people think that cutting down trees causes major floods, if that leads them to protect the environment? There are many solid reasons to conserve forests, and if stretching the truth a little helps to do that, some would argue it is no bad thing.

But there is a very good reason why we should not do this. Many governments have used the myth about deforestation causing major flooding to force poor farmers off their lands and away from forests. Yet most of those farmers have no other way of making a living. After the Yangtze river floods of 1998, for example, the Chinese government imposed a logging ban that put over a million people out of work. And several south-east Asian governments have used floods as an excuse to prohibit the traditional farming practices of ethnic minorities.

When people hear that logging causes floods they usually demand tougher laws and more arrests. That makes poor farmers' lives more difficult and forces them to pay higher bribes to forestry officials. It rarely benefits the forests. One reason the Dominican Republic has so few trees is that farmers are scared to plant them because they do not know if forestry officials will let them use them. Haiti's government reportedly wants to use the country's 8000 UN peacekeeping troops to keep farmers out of the forest.

That could convert one kind of catastrophe into another.

By David Kaimowitz, CIFOR. First published in the New Scientist June 19, 2004 (www.newscientist.com)