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Demands on the Atchafalaya River: Rice Farming in the Mermentau Basin of Southwestern Louisiana

Grand Prairie Rice Field, Arkansas

Rice farmers needing water: they need it on Grand Prairie in Arkansas where they are depleting the great Mississippi Aquifer; they need it in Southwestern Louisiana where numerous alterations to the landscape has changed the way freshwater flows through the Mermentau and Calcasieu Basins west of Vermillion Bay.

For the long story about rice farming On Grand Prairie in Arkansas, read The Mississippi. Essentially, rice farmers on Grand Prairie want to tap the White River, which runs along the eastern edge of Grand Prairie, and pump the water through a series of canals and into their rice fields. The pump is under construction at Du Vall’s Bluff along the White River, which could change the way water flows through the White River National Wildlife Refuge.

And like rice farmers on Grand Prairie who want to tap the White River for water for their fields, rice farmers in the Mermentau Basin want to tap the Atchafalaya River, some sixty miles or more away, depending on where you tap the Atchafalaya. It will require a series of pumps and canals to accomplish the task.

Last summer as the Mississippi and Atchafalaya carried the Flood of 2011 to the Gulf of Mexico, the rice farmers in the Mermentau Basin watched in frustration as their rice fields dried up in the Drought of 2011 and salt water intruded into them from the Gulf. Some farmers never even planted a crop.  And because they double crop and breed crawfish in their rice ponds, that source of income was also compromised.

The Mermantau and Calcasieu Basins form the Chenier Plain, which William Darby described to a “T” in 1818:

“The marsh between Vermilion bay and the Lake of Mermentau, has nothing to distinguish it from the other marshes of the country, except in its extent, which is about thirty miles square. This great expanse, though generally covered with grass, is not entirely denuded of trees. Near the sea-coast, a singular appearance attracts the attention. These are ridges, which rise above the common level of the marsh, are dry and solid land, clothed with live oak trees. These ridges appear to have once been the sea-shore, and to have been in succession abandoned by the surf, as others were formed by the same means; they all run in lines parallel to the shore, and are separated by lagoons, ponds or the marsh. It is extremely difficult to reach many of these islands; and as no adequate object presents itself to reward the trouble, they are visited but seldom by man. They are the undisturbed retreat of wild animals, deer, turkies, grouse, and perhaps the bear.”–William Darby, 1818

Sediment, pouring out of the Mississippi and flowing westward on longshore currents, came to rest in the successive ridges that form southern reaches of the Chenier Plain. We have disrupted that process of land building with the changes we have made to the Mississippi. There is less sediment spewing out of the river. What does flow out of the river is deposited in very deep water at the continental shelf, where it is unavailable for land building. We have disrupted the long shore currents with jetties. However, some sediment is flowing out of the Atchafalaya and being deposited on the eastern edge of the Chenier Plain, but not enough.

The Mermentau Basin breaks down into two parts: the northern wet prairie, where rice is grown, and the southern Chenier Plain, where ridges and wetlands alternate across the landscape.


Like Grand Prairie the northern reaches of the Mermentau Basin are wet prairie, where the low grade and heavy clay soil are ideal for rice farming, because clay prevents the water in rice fields from seeping into the aquifer below. And like Grand Prairie farmers have tapped the bayous that flow through the region and constructed a system of canals to deliver water to their rice fields. In doing so they have changed the way water flows through the region. 

The Corps of Engineers constructed a series of locks and water control structures on Grand and White Lakes to control water levels, convert them into freshwater reservoirs for rice irrigation, and prevent the intrusion of saltwater into the lakes. Continuing high water levels in the Lakes sub-basin drowned wetlands and shifted in the composition of plants. Where excessive flooding eroded the rims of Grand Lake and White Lake, the marshes behind and below the rims also eroded. Saltwater intruding along navigation canals killed freshwater plants, baring the soil that supported them. Tides washed the soil away, converting marshlands to open water.[i]

Then there is what we have done to the Chenier Plain itself: The construction of Louisiana Highway 82 in 1958 across the top of Pecan Island, through the marshes, and across the top of Grand Chenier Ridge, all in the southern reaches, stopped the natural flow of water from north to south. The system of thirty-two culverts and a dozen bridges, designed to allow drainage through the highway, never adequately replaced the natural drainage in the basin. By the turn of the twenty-first century most of the culverts had silted in or collapsed, turning Highway 82 into a dam across the basin, leaving the Lakes sub-basin to the north subject to constant flooding and the Chenier sub-basin to the south starved for freshwater.

The dredging of the Freshwater Bayou Channel, the Mermentau River-Gulf of Mexico Navigation Channel, and dozens of oil access channels allowed saltwater into the Chenier sub-basin marshes, which disintegrated. The introduction of suspended sediment into the western part of the sub-basin restored some of the marshes, but they were disappearing from the rest of the basin.[ii]

The Atchafalaya River is a distributary of the Mississippi. It carries 30% of the Mississippi all of the Red River to the Gulf of Mexico. It is the Mississippi “wannabe,” and Congress and the Corps of Engineers have worked hard to keep the Atchafalaya from capturing the full flow of the Mississippi. Louisiana’s coastal scientists are looking at the Atchafalaya as a source of fresh water for the declining coastal marches, particularly those in the western Terrebonne basin, where salt water is intruding into freshwater marshes. Already, Atchafalaya water is being directed into the Vermillion Basin, just to the east of the Mermantau Basin. A year ago Mermentau rice farmers watched their rice fields dry up as the Flood of 2011 drained away into the Gulf of Mexico, where it was useless for rice farming. Some of that precious freshwater did make its way into the western Terrebonne marshes, where, for the summer of 2011, it pushed salt water out of the marshes.

[i]             Clark, Darryl, and Mazourek, Joyce, Final Environmental Assessment, Freshwater Introduction south of LA Highway 82 Project (ME-16), Lafayette, Louisiana: U.S. Fish and Wildlife Service,  Marsh 2005, 1, http://lacoast.gov/reports/env/Hwy%2082%20ME-16.pdf.

[ii]             Louisiana Department of Natural Resources, Hydrologic Investigation of the Louisiana Chenier Plain, Prepared for the Louisiana Coastal Wetlands Conservation and Restoration Task Force, October 2002, 15, http://www.lacoast.gov/reports/static/HILCP_2.pdf;


One Response

  1. This is great information. I’ve slowly been pulling together a file for a trip across 82 and an exploration of the cheniers, and this is going to be very helpful. Now that you’ve pointed it out, that great swath of 82 across the landscape does look rather like a dam.

    I couldn’t find a definitive answer, but I’m assuming Mermentau Lake is called Grand Lake today.

    Another article I’ve found useful and interesting is a study of the effects of the 2005 hurricanes on the chenier forests and, hence, the migratory birds.

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