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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 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.
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