• The Mississippi: A Visual Biography by Quinta Scott

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MRGO and the Manchac Landbridge

 

Manchac Landbridge Between Lakes Manchac and Pontchartrain

Manchac Landbridge Between Lakes Manchac and Pontchartrain

Six streams flowed to Lake Maurepas including the Amite River and Bayou Manchac, a distributary of the Mississippi until Andrew Jackson built an earthen dam across its head in 1814. Pierre Le Moyne d’Iberville discovered that the Bayou could be a shortcut to the Gulf of Mexico in 1699. Before its closing overflow from the Mississippi carried freshwater into the swamps west of Lake Maurepas, and through the lake. Freshwater passed through Pass Manchac, a tidal pass through the Manchac Landbridge, and into Lake Pontchartrain. Lake Pontchartrain’s landbridges–the Manchac Landbridge and the East Orleans Corridor Landbridge between Pontchartrain and Lake Borgne–buffer storms. Like the Barataria Landbridge they isolate water of differing salinities from one another, saltwater from brackish water in the case of the East Orleans Landbridge, brackish water from fresher water in the case of the Manchac Landbridge, which extends from the mouth of the Tangipahoa River to the Bonnet Carre spillway. Fresh marshes line the western side of the Manchac Landbridge, intermediate marshes the eastern side.

 

Manchac Landbridge at Bayou Black

Manchac Landbridge at Bayou Black

When Europeans explored Louisiana the landbridge between Lake Maurepas and Lake Pontchartrain in the seventeen and eighteenth centuries, they found the canopy of the ancient cypress swamp spreading over a healthy understory and groundcover and supporting the indigenous creatures of the swamp. Commercial logging of cypress in the late nineteenth and early twentieth centuries decimated the forest by 1930. Loggers moved the felled trees out of the swamp along a series of ditches and canals, which changed its hydrology. Salt water seeped in along the canals; hurricanes and tropical storms flooded it with salty water.

 

Manchac Landbridge: Joyce Wildlife Management Area

Manchac Landbridge: Joyce Wildlife Management Area

Then came MRGO: The Corps of Engineers cut MRGO through the marshes west of Lake Borgne in 1963, the date of partial completion of the channel, salinity levels in Lake Pontchartrain rose, as much an average of 0.4 parts per thousand in Pass Manchac and more throughout the rest of the lake. Increased salinity stressed the cypress swamp on the landbridge. By 2007 what forest remained was a relic swamp, permanently flooded, stressed by saltwater, where young trees would not replace the dying cypress, but where intermediate and brackish marshes replaced the trees.

 

Manchac Landbridge at North Pass

Manchac Landbridge at North Pass

Nevertheless precarious stands of tupelo, and pond cypress still dominated the landbridge. Buttonbush and arrow arum covered the understory of the swamp. Bulltongue, groundsel bush, and marsh elder defined the intermediate marsh. Dwarf palmetto joined the groudsel bush and the marsh elder in the scrub-shrub habitat. Maidencane and cut grass dominated the fresh marsh. The various environments offered habitat to a variety of creatures. Largemouth bass and catfish preferred freshwater while red fish, speckled trout, and blue crab preferred saltier water. Sea birds fed in the lake, shore birds stalked its edge, raptors perched in its trees. Migrating songbirds dropped down into the forest in the spring for a rest. Alligators nosed through the marshes.

 

Lake Pontchartrain Shoreline, Manchac Landbridge at Frenier

Lake Pontchartrain Shoreline, Manchac Landbridge at Frenier

While the trees were in big trouble, ironically the marshes bordering the lake were not. Periodic releases of freshwater, sediment, nutrients from the Bonnet Carre spillway for flood control, at the southern end of the landbridge, have nourished the marshes over time. The Louisiana Department of Natural Resources has proposed formalizing Bonnet Carre releases for conservation when the Mississippi is running high and its water cool enough to prevent algae blooms in Lake Pontchartrain and freshwater could be directed into the Manchac wetlands through the Old Hammond Highway borrow pit. The Frenier Wetlands would benefit.

Closing MRGO to deep-draft ships at the Bayou la Loutre Ridge and constructing a sill in MRGO to halt saltwater intrusion would enhance all the wetlands surrounding Lake Pontchartrain.


    Kniffen, Fred B., “Bayou Manchac: A Physiographic Interpretation,” Geographical Review, Vol. 25, No, 3, July 1935, 462; Iberville, 81-84; Lake Pontchartrain Basin Foundation, Comprehensive Habitat Management Plan for the Lake Pontchartrain Basin, February 28, 2006, 35-36, 47, 52-53, http://www.saveourlake.org/pdfs/JL/CHMP_final_%2022706.pdf; McInnis, Nelwyn, and Bryan Rogers, ‘Priority Conservation Areas in the Lake Pontchartrain Estuary Zone,” The Nature Conservancy, http://www.nature.org/wherewework/northamerica/states/louisiana/files/estuary_zone_final_s.pdf; Coast 2050:  Toward a Sustainable Coastal Louisiana, The Appendices, Appendix C-Region 1 Supplemental Information, “West Manchac Landbridge,” “East Manchac Landbridge,” Baton Rouge, Louisiana Department of Natural Resources, 1999, 15, 17-19, http://coast2050.gov/reports/app_c.pdf ; The Nature Conservancy, Conservation Area Plan for the Lake Pontchartrain Estuary, Covington, Louisiana: The Nature Conservancy, Northshore Field Office, January 2004, 20-21.

 

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4 Responses

  1. Nice article. Haven’t been to Manchac in a while.

  2. Thanks, Quinta

  3. Quinta,

    Great information and photos. You make reference to the Old Hammond Highway borrow pit. I would like to find out more about the Old Hammond Highway and its history. Is the roadbed still there? Do you have any historical information or photos? Thanks.

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