To repeat what I wrote the other day:
“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.”
No sooner did President Obama pledge to restore the gulf coast, and no sooner did G. Paul Kemp suggest reallocating 80% of the flow at Old River to the Mississippi, than the bloggesphere lit up with discussions about reengineering the Mississippi to save the Louisiana coast. They’re thinking oil, not sediment.
They’re thinking using the principle of tidal exchange, freshwater from snow melt and spring rains overflowing the Mississippi banks and pushing out salt water that rode into the Louisiana marshes on winter tides when the river stayed in its banks.
The diversions from the Mississippi at Naomi, West Point a la Hache, and Davis Pond into the Barataria Basin, and the one at Caernarvon into Breton Sound have not succeeded in reestablishing tidal exchange, though the State of Louisiana has opened them wide to release water into the basins in the hopes of pushing out oil. And it worked to keep the oil at bay as long as the Mississippi was flooded, which is why Kemp made his suggestion.
Reengineering the Mississippi will entail changing the Mississippi River and Tributaries Project, the document that governs the Mississippi, the levees that line its banks, the dams that retain water in the uplands until a flooded Mississippi can handle it.
Changing every one of those trade-offs in the bold-print opening paragraph will gore somebody’s ox. Mississippi, Arkansas, and northern Louisiana have been building levees, since the first levee in New Orleans in 1721, three years after Bienville laid out the city on the high natural levee of the Mississippi. Ever since then the west side of the river has been competing with the east side of the river to see who could get the highest levee, because he who had the highest levee come floodtime won. His levee forced the flood on to the other side of the river. The Mississippi River and Tributaries Project settled that discussion and evened out the levees.
A year ago a pair of scientists at Louisiana State University published a paper that detailed the fact that most of the sediment the river carried at the beginning of the twentieth century is retained behind headwater dams in the uplands, 40,000 of them.
The Missouri, which once supplied the Mississippi with 60% of its sediment now retains it up river, starting at the Gavin’s Point Dam in Nebraska. South Dakotans and Nebraskans like their lakes and would be loath to give them up so Louisiana could have its mud. Never mind that the lakes are filling up with mud and you can see at Niobrara.
There is a plan to divert more water down Bayou Lafourche, in essence to reopen the bayou,which was closed in 1903 to stop flooding along it. Planners have not yet figured out how to get the silted-in bayou at accept the water they want to send down it. And people have settled along the pretty bayou since it was closed in 1903. They would have to move.
Reengineering the Mississippi to enhance Louisiana’s wetlands will be as much a political problem as an engineering problem. In the short term, changing the Mississippi/Atchafalaya ratio from 70/30 to 80/20 should be doable without a lot of hassle.