Water rushes out of the historic Independence Mill in Independence, Iowa, during flooding in July 2010, above. Below, an 1870 steamboat race on the Mississippi.  If recent events in Japan were not enough, the news of the past week has reminded us that nature can make our efforts to control it seem like nothing more than hubris. A historic swath of tornadoes has ripped across the South, and now a potentially major Mississippi River flood is gathering. The tornadoes have done their damage already. The rising waters of the Mississippi are about to test human judgment and engineering anew.  As of Friday, Maj. Gen. Michael Walsh of the U.S. Army Corps of Engineers was deciding whether or not to blow up a levee in Birds Point, Mo., and deliberately flood 200 square miles of fertile farm land in order to protect other property. It is a choice between having a controlled flood on a battlefield of his choosing or having the river breach the levee at some as yet unknown weak spot where the flood would be uncontrolled and could wreak havoc.  1937: A makeshift boat in Louisville, Ky.  Blowing the levee is not some wild idea drawn up on a napkin; it is one small piece of a carefully thought-out and engineered plan to control the immense forces of the Mississippi. The river drains 31 states and stretches from Olean, N.Y., to the Rockies, from North Carolina to Taos, N.M.  On the lower Mississippi—from Cairo, Ill., where the Ohio and Mississippi rivers meet, to the Gulf of Mexico—the floodplain, at its widest, stretches nearly 100 miles across. A great flood can easily fill the entire 35,000-square-mile area with water. The last time the Mississippi did so was in 1927, and the decision that Gen. Walsh now faces results directly from the plans developed to prevent such floods—and from our failures and oversights in implementing them.  The 1927 flood came as a surprise to no one. Precipitation over the entire river basin started in August 1926 and continued through the end of the year. In the last three months of 1926, all the gauges on the entire river system—on the Ohio, the Missouri, the Arkansas and the Mississippi itself—had the highest average readings ever recorded. On Christmas Day 1926, both Nashville and Knoxville—which sit on different rivers—flooded. On New Year's Day 1927, the Mississippi broke flood stage at Cairo, Ill. It remained above that level for 153 consecutive days.  Precipitation continued all spring, and the Mississippi and all its tributaries rose up in full rage. Ten discrete flood crests went down the river as different tributaries peaked, receded, then rose again. Pittsburgh, Cincinnati and Louisville flooded. Omaha and Oklahoma City flooded. So did virtually every city and town in between. From West Virginia to Oklahoma, people drowned.  But the greatest devastation visited the lower Mississippi, in Arkansas, Mississippi and Louisiana. A single levee break in Mississippi put water over the rooftops of Yazoo City, 50 miles to the east. Another break on the west bank allowed one to travel by boat over normally dry land from Vicksburg, Miss., 75 miles to the west to Monroe, La. As one eyewitness wrote, "For thirty-six hours the Delta was in turmoil, in movement, in terror. Then the waters covered everything, the turmoil ceased, and a great quiet settled down…. Over everything was silence, deadlier because of the strange cold sounds of the currents gnawing at foundations, hissing against walls, creaming and clawing over obstacles."  “Nature is perfect; engineers are not. If humans make a mistake against nature, nature will find and exploit it.”  In total, the river flooded the homes of nearly 1% of the entire population of the U.S. No remotely accurate death toll exists; the weather bureau officially said 313, but a professional in disaster management estimated at least 1,000 dead in Mississippi alone.  More significant than the devastation was the flood's aftermath. It sent Herbert Hoover—who was portrayed heroically in print, on newsreels and on radio for leading the rescue and rehabilitation efforts—to the White House. It severed the emotional bond between African-American leaders and the Republican Party and spurred enormous numbers of African-Americans to leave the South. Most importantly, it changed the way that people viewed government's role: Hundreds of thousands of people had lost everything through no fault of their own, and, for the first time, the nation as a whole seemed to decide that the federal government had a responsibility to help individual citizens in distress.  1927: A flooded highway in Illinois  The 1927 flood had riveted the nation's attention, and popular concern about a recurrence of the tragedy drove Congress to give more money to the Mississippi River and Tributaries Project than to any single national effort up to that time, except World War I. The flood also taught engineers a great deal about controlling rivers. In 1928 the Army Corps of Engineers applied those lessons as it developed a comprehensive plan to handle floods on the lower Mississippi.  The project resulted in levees that are by far the strongest in the country, along with other flood-control devices such as floodways, which are essentially parallel rivers that, by giving the river room to expand, lower the flood height in the main channel and thus ease pressure on levees. Birds Point, where Gen. Walsh must make his decision, is at the top of one such floodway; it has been used only once, in 1937.  Unfortunately, the lessons of 1927 were not applied with similar energy along the upper Mississippi, where the flooding of the past week has been the most severe, or elsewhere in the country.  1937: A submerged roller coaster in Cincinnati  The problem of protecting against river floods is complex. It requires a broad view of the river system as a whole, a narrow focus on local protection, and constant maintenance and monitoring down to almost infinitesimal detail. Nature is perfect; engineers are not. As recent experience in Japan demonstrates, if humans make a mistake against nature, nature will find and exploit it.  In the U.S., nearly all of the focus has been on local efforts at flood control, without considering the impact on entire river basins. The most glaring examples are the six great dams built on the Missouri River in Montana and North and South Dakota. Completed in 1963, they have provided billions of dollars in benefits to the High Plains in flood protection, hydroelectric power and irrigation.  But these dams have greatly increased the flood danger to Louisiana. They retain 150 million tons of sediment a year, which once poured down the Mississippi and into the Gulf of Mexico—more than the entire rest of the system sends to the Gulf. The loss of this sediment has been a major factor in the disappearance of 2,300 square miles of coastal Louisiana, land which provided a buffer protecting populated areas from a hurricane storm surge. As this buffer has melted away, New Orleans and the rest of coastal Louisiana have become far more vulnerable to hurricanes than nature made them.  There have been local problems, too. Levees are usually properly designed and constructed, but then slippage occurs. Several years ago the Corps of Engineers described the levee in Poplar Bluff, Mo.—which breached in many spots this week—as "unacceptable" because of poor maintenance. Nationally, the Corps has evaluated 237 levees and deemed 58 "unacceptable" because of similar failures.  1973: Filling sandbags in St. Louis  An even greater problem has been that, even in cities, the federal government builds levees to protect only against a "100-year flood," meaning a flood with a 1% chance of striking in any given year. This sounds like a high standard, but it is, in fact, the lowest in the developed world. Japan and the Netherlands, for example, protect against river floods to a 2,000-year standard for densely populated areas and a 200-year standard for rural zones.  The U.S. standard evolved as a political compromise when the National Flood Insurance Program was established in 1968. That program requires flood insurance for any homeowner with a federally backed mortgage whose home lies in a flood plain—but it waives the requirement if the home is protected by a levee that can hold a 100-year flood.  The Corps of Engineers wanted a minimum 500-year standard of protection for densely populated areas. But because of the high cost of building to this standard, the Office of Management and Budget opposed it. Developers lobbied for a 50-year standard. The resulting 100-year protection level was a classic Washington compromise.  2011: Homes engulfed by floodwaters in Missouri  But this standard exposes Americans to a much higher risk to life and property than they would accept in a retirement investment. The reality is that, over the 30-year life of a typical home mortgage, there is a 26% chance of a flood that exceeds the 100-year standard. Over the course of a century, there is a 63% chance that a flood greater than the 100-year standard will strike, a 26% chance of two such floods, and an 8% chance of three such floods. So it is hardly a statistical surprise that the upper Midwest suffered 100-year floods in 1993 and 2008.  In some cases the standard has actually increased flood risk. Developers push for levee construction, a levee gets built, homeowners move in thinking they're safe and don't buy flood insurance, since it's not required. Then a flood wipes them out.  And all of this assumes that the estimates for a 100-year flood are accurate. In many instances they are not. As Gerry Galloway, a retired Corps of Engineers general and flood expert, says, "It's a game of rolling dice. On the Nile you have thousands of years of data, but here we just don't have enough data. We really don't know what a 100-year flood is."  The difference between 100-year protection and a higher standard can be seen every year, when one compares floods on the lower Mississippi to floods everywhere else in the country. Levees along the lower river are strong enough to protect against waters generated by, approximately, a once-in-750-years storm. This area has not seen a levee breach since 1927.  By contrast, the 100-year (or less) protection for the rest of the country routinely generates disaster headlines. Fargo, N.D., has an almost annual wrestling match with the Red River. Two 100-year floods on the upper Mississippi ravaged the Midwest in 1993 and 2008. And last year in Rhode Island the Pawtuxet River devastated cities whose residents barely knew the river existed—much less that it was a threat—until it inundated their homes.  An 1870 steamboat race on the Mississippi.  Hurricane Katrina demonstrated the difference as well. All the old levees held; the newest ones, poorly designed structures that were supposed to contain that storm and were completed just six years before the storm, were the ones that collapsed.  We have the resources and technical knowledge to look at river basins in a comprehensive way and to construct far more effective flood control measures. But real protection costs money, which is in short supply in Washington these days. Even California, however, a state with one of the most desperate fiscal situations, has recognized the importance of better flood protection and now mandates that no new development can occur without protection against a 200-year storm. And it passed a $4.9 billion bond issue to help meet that standard.  No matter the standard, sooner or later a flood will come that exceeds it. But the system survives and continues to function even if water pours over the top. That's the difference between water just sloshing over the lip of a bathtub and the wall of the bathtub collapsing entirely, spilling out all the water within—which would be a catastrophe.  If we want to avoid a repeat of the upsetting scenes of the past week, we must start thinking about building our bathtub walls higher and sturdier.
The past week's raging waters along the Mississippi are hardly unprecedented. The great flood of 1927 was far worse—and we still haven't applied its lesson.