Dealing with the Deluge
Around the world, unruly rivers have long driven a hard bargain. In exchange for rich soil, irrigated land and convenient transportation, they have forced floodplain dwellers to deal with an occasional washout. Engineers have labored for thousands of years to lessen the risk, but their attempts at managing Mother Nature have been mixed -- often resulting in as much failure as success.
The Yellow River
China has had a particularly long and terrible history of flooding. In the last 2000 years, the Yangtze River has flooded more than 1000 times. But it is the Hwang Ho or "Yellow River" that has been responsible for China´s most catastrophic floods. Three thousand miles long, it begins high above sea level in the northern mountain province of Qinghai and ends at the Yellow Sea. Westerners have dubbed it "China´s Sorrow," because over the centuries it has killed more people than any other river in the world. In 1887 flooding killed nearly two million people, in 1931 the death toll was almost four million, and in 1938 it was almost one million.
Much of the problem stems from the high silt content of the river -- in some stretches as much as 60% by weight. Millions of tons of yellow mud choke the channel, causing the river to overflow and change course. In its lower reaches, the river bed has actually become higher than the level of the surrounding countryside. Water is held in by dikes of ever increasing height, some reaching 30 feet and more.
Attempts at controlling the Yellow River were begun as early as the third century B.C. An engineer named Yu came up with the idea of dredging the river to encourage the water to flow in its proper channel. Yu was made Emperor of China for his contribution, but managing the river´s silt would continue to be an ongoing challenge.
Over the years, the Chinese have tried to control the Yellow River by building higher levees, digging channels and building dams. Dams have tended to be the most helpful in controlling floods, but the river´s thick silt has clogged many of them. Currently, the Chinese are constructing a massive new dam called the Xiaolangdi Multipurpose Dam Project. Boasting 10 intake towers, nine flood and sediment tunnels, six power tunnels and an underground powerhouse, the structure may finally mitigate "China´s Sorrow."
The Nile River
The Egyptians have had a very different relationship with the Nile. For thousands of years, they referred to its annual flooding as the "Gift of the Nile."
Each summer, like clockwork, the river would take possession of a strip of land on either side of its banks. When the water receded, a very thin, evenly spread layer of black mud was left behind. Farmers would immediately plant their crops -- never needing fertilizers because the flood soil was so rich.
This narrow strip along the Nile, together with the delta at the river´s northern mouth, is the only farm land Egypt has. Though it totals only three percent of the county´s land, it has provided ample food for thousands of years. But recently, a population boom has forced Egyptians to increase their agricultural output.
In 1970, they completed the Aswan High Dam, which stretches across the Nile 600 miles south of Cairo. The dam has effectively stopped the river´s annual floods by trapping its waters in a reservoir that is slowly released during the dry season.
Now farmers along the Nile plant crops year round. In fact, the area has become one of the most intensely cultivated pieces of land in the world. Because the Aswan Dam traps 98% of the river´s rich sediments and prevents them from flowing downstream, farmers along the Nile must now use large amounts of artificial fertilizer. Another negative side-effect of the dam is that the Nile delta is no longer being built up by the river sediments. As a result, this important agricultural area is now struggling with erosion and dangerously high levels of soil salinity.
The Mississippi River
Beginning as a trickle in northern Minnesota, the Mississippi flows nearly 2,400 miles south to the Gulf of Mexico. Along the way, it draws water from thousands of tributaries across 31 states and two Canadian provinces. The Mississippi River valley is one of the world´s most fertile, providing food and jobs for millions. The river is also a superhighway for commerce, moving 50 billion dollars worth of goods through America´s heartland every year.
Flooding is nothing new for the Mississippi, which has regularly overrun its banks for tens of thousands of years. What has changed over the past 100 years is the number of people effected by these natural upheavals.
After a disastrous flood in 1927, the U.S. Army Corps of Engineers was charged with task of taming the mighty Mississippi. In building the longest system of levees in the world, the Corps successfully minimized flooding and improved the river´s navigability. This dramatic achievement spurred millions of Americans to move onto the floodplain, where the soil was fertile and the property was cheap. As a result, much of the river´s bordering wetlands have been lost to agriculture and construction.
Today, the Mississippi is outfitted with 29 locks and dams, hundreds of runoff canals, and miles and miles of levees. Most years, the system works remarkably well, but the flood of 1993 washed away the illusion of complete control. Fully 80% of private earthen levees in the river basin failed. Most federal levees held, saving lives and land -- but sent torrents of water towards less protected field and towns.
The ´93 flood rekindled an old debate: what to do about the loss of the river´s natural floodplain. Originally, the waters of the Mississippi spread over many thousand square miles of lowlands. Unlike cultivated farmland, wetlands possess a sponge-like quality that absorbs excess water in times of flooding. Today the Mississippi is largely confined to its immediate channel, and the increased runoff from the surrounding land makes the river even more vulnerable to overflowing. Much of the public debate has focused on encouraging farming practices that improve soil absorption properties and allowing some of the land along the Mississippi to revert to its former wetland state. But even with these enlightened practices, there is truly no way to eradicate flooding of the Mississippi -- or any other river.
Photos: ( 2) NASA; ( 3) WGBH Educational Foundation.
