This is a difficult story to tell without Pshop to create diagrams, but there are many websites not created on an iPhone that describe the place and events in more detail. The Wikipedia entry is a good place to start, and this has more detail.
When I first read John Mcphee’s The Control of Nature I was enthralled by his description of the battle to prevent the Mississippi following its natural inclination to shorten its route to the Gulf of Mexico by moving west into what is now the Atchafalaya River. Over the last fifteen years I’ve re-read McPhee, pored over Google search results and satellite images, and bored A. with exclamations and explanations. Today I saw the real thing.
We followed the Mississippi south from Vidalia on Hwy 15, which frequently contours the side of the main levee (the steep slope on the left).
Looking west across the floodplain.
The landscape reminded us of the English fenland: flat, fertile agricultural land, but here the soil is fine brown silt, darker in areas richer in organic matter where the natural vegetation was cleared more recently, or floods have not deposited silt for some time.
In the early 19th century a large westward meander of the Mississippi known as Turnbull’s Bend was lengthening the journey time of river traffic. The meander had intersected the Red River, which flowed into the bend from the north, while a distributary (the opposite of a tributary: one of the streams into which a big river divides as it enters its delta) of the Mississippi, the Atchafalaya, flowed south out of the bend. In 1860 Col. Henry Shore decided to shorten journey time by cutting the neck of the meander. But instead of flowing nicely down its own straightened course, the Mississippi began to send water west across the lower half of the meander, transferring more and more water to the Atchafalaya. Over the next century it became clear that if nothing was done, the Mississippi would transfer all its flow to the Atchafalaya, leaving the city and port of New Orleans without its river, while threatening Morgan City and other settlements along the Atchafalaya. In 1958 the U.S. Army Corps of Engineers completed the Overbank Control Structure of levees to separate the rivers, plus a dam and spillway allowing the Mississippi to overflow into the Atchafalaya at times of high water.
The downstream side of the spillway. Each bay is a gate that can be opened; we think the concrete blocks and pool are intended to minimise major turbulence that could damage the structure.
In 1963 they completed the Old River Control Structure, which comprises an earth dam across the lower meander, blocking the Mississippi’s free flow down the Atchafalaya, and a new Outflow Channel connecting the two rivers that is controlled by the Low Sill Structure.
The ‘blocks on legs’ (not the crane arm visible in the picture) are rail-mounted cranes that travel along the structure to raise and lower the gates controlling flow through the structure. We were very lucky: while we were watching they opened one of the gates to allow water to flush sediment out of the structure and the channel.
There’s a yellow block at each side of the crane. Each block locks into, then lifts, one side of the gate – there are four gates stacked in each section of the structure. Only the top gate in one section was lifted; we ran to the other side of the road to see the first surge of water.
Huge carp were jumping out of the surging water, probably trying to release painful air pressure in their swim bladders after being pulled through the gate.
South of the Low Sill Structure is the Auxiliary Structure, an additional channel and set of gates built to take some of the Mississippi flow after the Low Sill Structure almost failed in 1973.
A. was as keen as I was; the man working one end of the Low Sill crane was obviously happy to tell two mad Brits interesting things about the Structure, especially as most of the people who live here – and rely on it to protect their property – drive across without showing any interest at all. We made up for that!
Red River Fish! Fish! (There were two kinds of gyrinid beetle, too.)
More pines and grass. The air is less humid. People speak with a different accent, more Texas.
I’m beginning to truly understand that it’s geology/geomorphology plus CLIMATE that makes landscape.
Tomorrow we head south to Morgan City on the Atchafalaya. Road kill wildlife score so far: three armadillos, two raccoons, a vulture. I hope to report live alligators and more in the next two or three days.