World War I produced a major resurgence in river trade. As other lines of transport became congested, the river was recognized as an increasingly valuable asset. With federal initiative, new barge lines were organized, and by 1931 the annual barge traffic moving along the river was twice the volume moved in any single year during the previous century. In 1907, for instance, the steamer Sprague established a new world record for size of tow. Its raft of 60 coal barges weighed 67,307 tons and covered an area of 6.5 acres (2.6 hectares).

Sprague’s record is unlikely to be matched by any other paddle wheeler, for the remaining steamboats are mostly showpieces, and the modern Mississippi towboat is of design radically different from its forebears. Screw-driven and diesel-engined, the modern towboat is made fast to the stern of its tow. Ahead stretches a rigid platform of barges as much as 1,500 feet (460 metres) long and with a designed draft of 9 feet (2.75 metres). Most barges are built for specific cargoes. For dry cargo they average 1,500 tons capacity and measure 195 feet (60 metres) long by 35 feet (10 metres) wide; for liquid cargo the proportions are about 2,500 tons capacity and 295 feet (90 metres) by 50 feet (15 metres). To aid navigation, towboat captains have at their disposal electronic depth finders, radar, contraguide rudders, global positioning systems, a sophisticated system of riverbank lights and markers, and a radio telephone to warn other river users of their approach in narrow passages. Among the cargoes to be carried along the river in this fashion since about 1960 are the booster rockets for space research, which are so bulky as to be unsuitable for any other mode of transport.

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Commercial use of the Mississippi waterway has shown sturdy growth. Leading cargoes, by bulk, are petroleum and derivative products, coal and coke, iron and steel, chemicals, sand and gravel, crushed rock, and sulfur. An increasing emphasis on bulk handling inevitably has meant the rapid growth of a few key port cities at the expense of their rivals.

Responsibility for overseeing the maintenance and improvement of the Mississippi as a commercial waterway, as well as the related tasks of flood control and bank protection (see below), falls upon the Mississippi River Commission. Created by act of Congress in 1879, the commission has supervised a massive program of river work that has profoundly reshaped the character of the Mississippi. The main stages of the navigation-improvement program include a channel 9 feet (2.75 metres) deep and 250 feet (75 metres) wide at low water between Cairo, Illinois, and Baton Rouge, Louisiana, authorized in 1896; a widening of this channel to 300 feet (90 metres), approved in 1928; and its deepening to 12 feet (3.65 metres), was begun in 1944. In addition, a ship channel 45 feet (14 metres) deep from Baton Rouge to the Gulf of Mexico has been under construction since its authorization by Congress in 1945. Farther upriver, the outstanding achievement has been the construction of 29 locks and dams on the upper river to take the 9-foot channel upstream to Minneapolis–St. Paul. Connecting into the Mississippi is a vast complex of related waterways—from the Intracoastal Waterway in the south, stretching between Florida and the Mexican border, to the channels that lead up the Ohio to its tributaries and through the Illinois waterway to the Great Lakes and St. Lawrence Seaway.

Flood control

Flood control along the river dates to the foundation of New Orleans in 1717 by the French, who built a small levee to shelter their infant city. Over the next two centuries a complex array of riverbank structures was erected along the river to contain or divert floods. But it was not until after the catastrophic flood of 1927 that the federal government became committed to a definite program of flood control. The target has become an integrated flood-control system able to master a “project flood,” the largest theoretical flood expected along the river. This program has altered the face of the river even more than the navigation program—with which it is linked—has altered its bed.

In principle, the Mississippi’s floods are either constricted by levees, hastened out of danger zones by floodways and improved channels, dissipated down spillways and into reservoirs, or starved by the impounding of tributary floods. From Cape Girardeau, Missouri, to the Gulf of Mexico, the river is virtually “walled in” by a vast line of main-stem levees. This concrete barrier has, incidentally, isolated the river from much of the surrounding countryside; hence, many former riverbank towns are now severed from their natural setting. In the event that the main levees are threatened, the excess floodwater drains away down spillways (e.g., from north of New Orleans it is diverted past the city by spillways leading through Lake Pontchartrain to the Gulf of Mexico) or bursts “fuse plugs,” the specially weakened sections of levee leading to harmless floodways or reservoirs. A major example of this type of floodway occurs at New Madrid, Missouri, just south of the Ohio confluence. At times the system is overwhelmed, such as in 2005 when low-lying New Orleans was flooded after the levees holding back Lake Pontchartrain and the Mississippi River were unable to contain the storm surge waters accompanying Hurricane Katrina. Another such occurrence was in the spring of 2011 when snowmelt and heavy rains produced record high flood crests on the Mississippi that forced the opening of numerous floodgates and sluiceways and produced massive flooding of farmland and riverside towns.

Elsewhere a massive program of bank strengthening undertaken by the U.S. Army Corps of Engineers, using mattresses of concrete slabs, has reduced lateral erosion and increased channel stability. The careful positioning of underwater dikes to deflect the current, the cutting through of oxbow bends at their necks (i.e., straightening the channel), and a continuous dredging schedule have all helped to reduce flood levels and assist navigation. Although the system operated successfully against the high-water levels of 1945 and 1950, it could not contain the flood of 1973, the record inundation of 1993 of the upper and middle Mississippi and the lower Missouri, or the massive spring flooding of 2011. No matter how large, once flood-control reservoirs are full, they serve no additional function, since at that point all the water that enters must be immediately let through.

Thus, the Mississippi flood-control program is not without its problems. It has been increasingly necessary, now that the river is caught behind the main-stem levees, to attend to the isolated tributaries. In some cases, costly pumping stations have been installed to lift the impounded water over the levees and into the main river. More dramatic has been the penalty for using the Atchafalaya River as a convenient spillway for floods on the lower river. The Mississippi threatened to divert permanently into this secondary channel, inundating the lower Atchafalaya, bypassing New Orleans, and rendering useless millions of dollars of flood-control works and docks. Only at the cost of a vast and complex system of locks and barrages has the danger been averted. It has become increasingly clear that the river’s delicate hydrology has been disturbed and that the program of river work must continue for many years. Perhaps because of the impacts of the 1993 Mississippi floods and Hurricane Katrina in 2005 and the importance of the long-term health of the riverine environment, flood-control management and prevention strategies continue to be reviewed and evaluated. Presumably, future floodplain management strategies will be designed more to limit land use on flood-prone areas to those kinds of uses that can tolerate flooding rather than to prevent or minimize flood damage in those areas by way of engineered structures.