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.

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.