Seven years ago, it was the Mississippi. Three years ago, it was the Red River. Two years ago, the Yangtze River in China. This year, Mozambique.
Recent years have seen a, well, a flood of devastating floods all around the world. And they’re getting worse. In 1998, total losses from weather-related natural disasters, including floods (the most destructive type of weather-related disaster) reached $90 billion U.S.–more than the toll for the entire decade of the 1980s.
There’s nothing very complicated about why rivers flood. If a river can’t handle the load of water it’s required to carry, it must rise. If it rises high enough, it will rise above its banks. The faster water runs from the watershed into the river, the faster the water will rise, and the higher the flood will be.
When a Saskatchewan river floods, it’s usually because of a rapid melt of heavy snow. With the ground still frozen, the water runs rather than soaking into the soil. This results in a large, fast flow of water into rivers, which quickly overrun their banks.
That’s a natural occurrence. Many of the recent deadly floods around the world, however, have been made worse by human activity. In Mozambique, 99 percent of the trees in the watershed of the Limpopo River, the major source of the flooding, have been removed over the past 50 years. In China, the Yangtze River watershed has lost 85 percent of its forest cover in the last few decades–and historical records show that centuries ago, the Yangtze flooded Hunan province only once every 20 years, whereas now it floods nine years out of 10.
Trees prevent floods in a number of ways. To begin with, some rainfall stays on the leaves, and may evaporate directly into the air. Leaves also break up the fall of raindrops, resulting in less impact on the soil, and less erosion. Tree roots absorb water from the soil, allowing it to soak up more rainwater, and also hold the soil in place, again, reducing erosion.
Erosion is bad because as dirt flowing into river beds clogs and narrows them. This means they can carry less water and the water they do carry moves faster (just as the gentle flow of water in a water hose suddenly becomes a rapid spray if you narrow the opening with your thumb).
Deforestation isn’t the only human activity that contributes to floods. Roads and ditches do, too. Besides being made of compacted soil or pavement, roads funnel water through culverts, which results in more water reaching rivers and, again, contributes to erosion. Ditches in farmland have the same effect of funneling the water more quickly toward the river.
Urban sprawl contributes to flooding, too, because, of course, nothing soaks into pavement. Careful developers try to avoid channeling water into streams, use as little pavement as possible, and build catch basins which let water seep into the soil.
The draining of wetlands also contributes to flooding. Wetlands act like giant sponges that can soak up enormous amounts of water, then release it slowly downstream. Essentially, wetlands are giant catch basins, with a large inlet and a small outlet.
For most of the last century, the standard method of preventing floods has been building dams or levees. These work fine in ordinary years–but when the extraordinary happens, like the Red River flood of 1997 or the Mississippi Flood of 1993, they actually make things worse. For one thing, the levees artificially narrow the river, which means that when a huge amount of water hits the river, it rises faster. And when the flood overtops the levee, or the levee breaks, the resulting destruction is likely to be worse than it would have been without the levee, because the levee has encouraged people to build and live in the river’s natural floodplain.
Since the 1993 flood, some levees on the Mississippi have been torn down, restoring 50,000 acres of floodplain. But hundreds of thousands of people still live in the river’s historical floodplain–and sooner or later, the Mississippi is going to reclaim it.
As long as people insist on living in the floodplains of rivers around the world, floods are going to cause havoc. And all the efforts made to tame rivers so people can live in those dangerous areas may only, in the end, make the havoc worse.
Mozambique may have been the latest country to suffer devastation from a flood, but it won’t be the last.