“Artificial levees, extending clear to the Gulf of Mexico, may have made human habitation of the delta south of Cape Girardeau possible, but prevented the Mississippi from refreshing its marshes with its sediment when it did flood. Revetments built of concrete mats may have stabilized the navigation channel, but reduced erosion of the banks, a source of sediment in the marshes. James Eads’ jetties at the mouth of the river may have allowed the river to cut a thirty-foot navigation channel through the sandbar blocking the South Pass of the modern delta, but delivered sediment carried by the river to very deep water at the continental shelf, where it washed away, never to be used for marsh building. Closure of the old distributaries of the river may have prevented flooding in the bayou country of Louisiana, but cut the flow of Mississippi sediment to the coastal marshes. Channel dams may have made navigation on the Upper Mississippi profitable, but they retained its sediment north of Alton, Illinois. Dams on the Missouri, from which the Mississippi drew sixty percent of its sediment, did the same. In short at the beginning of the twenty-first century, the coastal marshes south of New Orleans received eighty percent less sediment than they had at the beginning of the twentieth and eroded away.”
That in a nut-shell is how we have reengineered the Mississippi since the Corps of Engineers took control of the river in 1819. And every change we have made to the river has been a trade off, good for one constituency, bad for the Louisiana Coast.
The first damaging project to the Louisiana coast has to be the Mississippi River and Tributaries Project, designed, ironically to reduce the size of a Lower Mississippi River flood as it streamed past New Orleans. And, the Mississippi River and Tributaries Project is still an active project, one that Congress and add to or subtract from anytime. That could be a plus, but to date has been an ongoing minus.
In the wake of the Flood of 1927, Congress passed the Flood Control Act of 1928 and authorized the Corps of Engineers to reengineer the Mississippi so such a flood would never inundate the Lower Mississippi Valley from valley wall to valley again.
First came the levees from Cape Girardeau, Missouri clear to Venice, Louisiana on the west bank; from the mouth of the Ohio River to Bohemia on the east bank. While they did make human occupation of the Mississippi, Arkansas, and northern Louisiana delta lands possible, they prevented the Mississippi from flooding the Louisiana coastal lands with freshwater and sediment, disrupted tidal exchange–freshwater to the marshes in the spring which pushed out winter’s salt water intrusion, and made the Louisiana wetlands tidal-dominated rather than river dominated.
Then came the dams on the headwaters of the tributaries, only a few at first–on the St. Francis in Arkansas and the Yazoo in Mississippi, but more and more in the last 80 years since the Mississippi River and Tributaries Project governed the river from Cape Girardeau south. The dams retained sediment in the uplands and had their own management problems. The dams on the Missouri, built for an assortment of reasons–navigation, flood control, recreation–but mostly because the Corps of Engineers could could and Congress approved. At the beginning of the twentieth century the Missouri supplied the Mississippi with 60 % of its sediment and even today it is possible to go on Google earth and see the muddy Missouri streaming into the Mississippi, the Missouri flowing on the west, the Mississippi on the east for several miles.
Then came the floodways–the New Madrid, the Atchafalaya, and the Bonne Carre–designed to siphon floodwater from the Mississippi. In a project flood, 3 million cubic feet per second, the Atchafalaya would divert 1.5 million cfs to its floodway, and the Bonne Carre 250,000 cfs to Lake Pontchartrain, leaving 1,250,000 cfs to stream past New Orleans to the Gulf of Mexico.
Then came the revetments, designed to stabilize the navigation channel, but which armored the banks with concrete, thus preventing the meandering river from shaving sediment from its banks and carrying it down stream to Louisiana.
Then came the Old River Control Structure, which rations 30% of the Mississippi to the Atchafalaya, built because it looked as though the Atchafalaya would capture the full flow of the Mississippi, send it to the Gulf of Mexico, drown Morgan City, and leave Baton Rouge and New Orleans without a deep-draft river for its ports, because the Mississippi would, in time, silt in.
James Eads’ navigation channel through Southwest Pass, which allowed deep-draft, ocean-going ships into the Mississippi–only became a detriment to nourishing the wetlands when the levees, revetments, dams, and Old River Control were in place, because then it delivered whatever sediment was left in the water column to deep water in the Gulf of Mexico.
For a complete discussion of the Mississippi River and Tributaries Project and how we have changed the Mississippi and how we are learning to manage the river we have created, read The Mississippi: A Visual Biography, available from the University of Missouri Press or at your local library. It’s more than pretty pictures, but the pictures are beautiful and there are 200 of them.