Once people began constructing permanent buildings on floodplains, they became vulnerable to the destructive forces of floods. Moving water is extremely powerful because of its weight, and the force that it concentrates on anything in its path. In addition to simply pushing against structures, flood waters often erode away the soil and stones surrounding building and bridge foundations, weakening them. Floods also bring mud; lots of it. And they leave it everywhere in damp, reeking deposits crawling with insects and other creepy life forms. If you have ever experienced flood waters in your home, you will understand why the cleanup from floods is so appalling.
Floods are the most destructive natural events in the United States because about half of the nation’s cities and towns are built along rivers! As mentioned above, floods increase in catchments that have been deforested and paved over. Sometimes catastrophic floods are caused, indirectly, by human built structures such as dams and levees. For example, on May 31, 1889, a dam failure released the Conemaugh River to deluge Johnstown, Pennsylvania with a 36-foot wall of water. In the flood and ensuing fires that broke out, 2,300 people were killed.
In the fatal spring flood of 1927, after weeks of incessant rains, the Mississippi River went on a rampage from Cairo, Illinois to New Orleans, inundating hundreds of towns, killing as many as a thousand people and leaving a million homeless. After the great flood of 1927, the U.S. Army Corps of Engineers built miles of levees and 29 locks and dams on the Mississippi to try to control the rivers’ floods and improve navigability. Millions of people settled on the fertile floodplain behind the levees, and much of the former floodplain wetlands area was turned to agriculture.
Without the sponge-like wetlands to absorb excess water, great floods on the Mississippi became truly forbidding. Another great flood occurred in 1993. In the 1993 flood, the river carried only about one-third the volume of water as in the great flood of 1927, yet 80% of the private earthen levees in the river basin failed. The federal levees that did hold kept the river out of its floodplain, but also caused the water to “pile up” upstream, because instead of a broad floodplain, the water had to move through a relatively narrow gap between levees. A portion of the flood wall at St. Louis almost overtopped, but several levees broke downstream, causing the river to fall at St. Louis and saving this populated area. Although it was unfortunate that the levees broke downstream, the damage was much less in this agricultural area than it would have been in the city. This great flood lasted the entire summer growing season, and killed forests throughout the basin.
Now people are reconsidering the Mississippi River again. Humans need to learn how to work hand-in-hand with natural flood-dissipation processes to protect their towns and property.
The deadliest river in the world is not in the United States, it is in China. The Yellow River, “China’s Sorrow,” carries a huge load of silt from its headwaters in the mountains of Qinghai to the Yellow Sea. This silt accumulates in the channel, causing the river to go out of its banks and change course frequently. Over the centuries, the death toll from the Yellow River has been staggering: nearly 2 million people in 1887, almost 4 million in 1931, and another million in 1938. The Chinese have tried many engineering solutions to control the Yellow River—dams, levees, dredging—but it continues to flood, and take lives, on a regular basis. In the Extensions section of this lesson, you can take a virtual voyage down the Yellow River with dozens of excellent photos.[Renee do you want to leave this last sentence as is? The kids aren't going to see that section are they?]
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