The Mississippi River floods in April and May 2011 are among the largest and most damaging along the U.S. waterway in the past century, rivaling major floods in 1927 and 1993.

David Leonard, 16, helps carry sandbags through a backyard of a house as water from the Morganza Spillway is expected to threaten the home in Stephensville, Louisiana May 15, 2011. [China Daily]

In April 2011, two major storm systems dumped record rainfall on the Mississippi River watershed. Rising from springtime snowmelt, the river and many of its tributaries began to swell to record levels by the beginning of May. Following the Great Mississippi Flood of 1927, much effort has been invested in building defenses to withstand a flood of three million cubic feet per second just upstream from the Old River Control Structure.

The US Army Corps of Engineers refers to this design goal as the "project flood." As of 11 May 2011 the expected flow will be on the high side, but still within that maximum capacity, assuming everything works as expected.

The U.S. Army Corps of Engineers on Saturday opened two of the 125 floodgates at the Morganza Spillway 45 miles northwest of Baton Rouge, and opened two more on Sunday. Opening the floodgates - a move last taken in 1973 - will channel water away from the Mississippi River and into the Atchafalaya River basin.

That will take the floodwaters toward homes, farms, a wildlife refuge and a small oil refinery but avoid inundating New Orleans and Louisiana's capital, Baton Rouge. Over geologic time, the lower Mississippi River has used different pathways as its main channel to the Gulf of Mexico across the delta region.

Left to its own devices, about every 1000 years a new main channel forms through the natural process of delta switching. Either of two new routes — through the Atchafalaya Basin or through Lake Ponchartrain — might become the Mississippi's main channel if control structures are overtopped or severely damaged during a severe flood, such as the present 2011 event.

If the Mississippi changes course to the Gulf, it would probably happen at one of four locations, three involving the Atchafalaya, and one involving Lake Ponchartrain. The expected 2011 flood flow approaches but is still below the maximum design capacity of the flood-control system.

The Old River Control Structure sits at the normal water elevation and normally diverts 30% of the Mississippi's flow to the Atchafalaya River. There is a steep drop away here from the Mississippi's main channel. If this facility were to fail during a major flood, there is a strong concern the water would scour and erode the river bottom enough to capture the Mississippi's main channel.

The structure was nearly lost during the 1973 flood, but repairs and improvements were made after engineers studied the forces at play. In particular, the corps made many improvements and constructed additional facilities for routing water through the vicinity. These additional facilities give the corps much more flexibility and potential flow capacity than they had in 1973, which further reduces the risk of a catastrophic failure in this area. A failure of this system resulting in a new river pathway will have major economic and social effects.

About 2,500 people live in the spillway's flood path and 22,500 others, along with 11,000 buildings could be affected by backwater flooding - the water pushed back into streams and tributaries that cannot flow normally into what will be an overwhelmed Atchafalaya River. The only other way for the water to flow is through New Orleans and expose over a million people to risk.

Some 3,000 square miles of land could be inundated in up to 20 feet of water for several weeks. When flows peak around May 22, the spillway will carry about 125,000 cubic feet per second, about one quarter of its capacity. Some 18,000 acres of cropland could be flooded as waters rise, hitting their crest in about a week and remaining high for several weeks.

In addition to threatening densely populated areas, lower Mississippi flooding was a risk for as many as eight refineries and at least one nuclear power plant alongside the river.