The Houma Navigation Canal is Terrebonne Basin’s answer to MRGO, with similar results. When I first visited the Caillou Marshes in the Terrebonne Basin in 1995, I crossed the road between Bayou Grand Caillou and Bayou du Large and came upon this dead cypress forest, the first I had seen.
Terrebonne Parish, Louisiana
Cross the basin between the Bayou Grand Caillou and Bayou du Large along the Falgout Canal road and you cross a dead cypress swamp. Some say the construction of the Houma Navigation Channel, which runs through it, was the culprit. Its spoil banks isolated the swamp from its source of freshwater and sediment, leaving it vulnerable to the influx of tides washing in saltwater from the Gulf of Mexico. Some say it died off after Hurricane Betsy drowned it in salt water in 1965. Some lay it at the doorstop of the oil industry. None of that helped.
In 2005 when scientists from the U.S. Geological Survey who studied aerial photographs of marshes in the Lafourche Delta made in 1956, they found an unbroken span of interior marshes, bridging the space between Bayou du Large and Bayou Grand Caillou to the east. By 1968 oil canals with their spoil banks crisscrossed the wetlands, the marsh was beginning to fragment, small ponds were forming the marsh, creating submerged marshes. Within six years the areas of submerged marsh were well defined and the ponds had grown larger. Land-loss hot-spots, which defined the edges of the ponds, persisted into 1990s. Marshes went through cycles of submersion and exposure as water levels varied. Finally, hot-spots disappeared, submerged in water that was often over three feet deep. What remained were high-marsh islands, natural levees, and spoil mounds.[i]
Bayou du Large/Bayou Grand Caillou: Caillou Marshes
Terrebonne Parish, Louisiana
In 1995 you could turn a corner onto slightly higher, slightly drier land next to the levee that the Falgout Canal road runs on, and you would have come upon a screen of healthy cypress trees. Within eleven years, by the fall of 2006, all but seven trees had died. This is the head of the Caillou Marshes, which lie between Bayou du Large and the Houma Navigational Canal and Bayou Grand Caillou and extend from the Falgout Canal to the Gulf of Mexico.
Where the cypress died, locals, who remember 1949, tell of a fresh, floating maidencane marsh, hosting thickets of wax myrtle and a stand of cypress. It covered forty percent of the 87,079-acre marsh. A band of brackish marshes separated the fresh wetlands to the north from the saline marshes to the south along the edge of the gulf. The Houma Navigation Channel relayed salt water from the Gulf of Mexico and bled it into the freshwater marshes, assisted by the grid of trenasses, small access bayous, dredged by anglers and hunters in the 1950s.
The lost was catastophic. By the mid-1970s the fresh marshes had converted to open water. By 1988 thirty-four percent of the area had converted to saline marshes and thirteen percent to brackish marshes. The rest, more than fifty percent, was open water with 13,385 acres or 25.8% of wetlands lost in the northern and central portion of the area. If this rate of loss continued, by 2050 what was left in 1990 would be gone.
At the beginning of the new century the only high spot for these marshes was the possibility that the delivery of fresh water from the Atchafalaya to the Intracoastal Canal and then the Houma Navigation Channel might increase the quantities of riverine water to these wetlands and prevent further loss. Stabilizing the banks of the channel would make this possible.
Equally important to stabilizing the marshes would be finding a way to transfer fresh water and sediment from the Atchafalaya to the wetlands, possibly through the lower Penchant tidal marshes east of the Bayou du Large ridge.
[i] Coast 2050: Toward a Sustainable Coastal Louisiana, The Appendices, Appendix E-Region 2 Supplemental Information, Baton Route, Louisiana Department of Natural Resources, Caillou Marshes, 1999, “Caillou Marshes Unit,” 37-39, http://coast2050.gov/reports/app_e.pdf; Coastal Impact Assistance Program Project Nominee Fact Sheet, “Falgout Canal Freshwater Enhancement Project,” February 2006, http://dnr.louisiana.gov/crm/ciap/project-proposals/Region3/Terrebonne/Official/3C-T.%20falgout%20canal%20FW%20enhancement.pdf ; Louisiana Department of Natural Resources, “Draft: Louisiana Coastal Impact Assistance Plan,” February 2007, 186-88, http://dnr.louisiana.gov/crm/ciap/draftplan.2007.02.01.pdf.
[i] Morton, Robert A., Bernier, Julie C., Barras, John A. and Ferina, Nicolas F., “Rapid Subsidence and Historical Wetland Loss in the Mississippi Delta Plain: Likely Causes and Future Implications,” U.S. Geological Survey Open-file Report 2005-1216, 1, 12-14, 20, http://pubs.usgs.gov/of/2005/1216/ofr-2005-1216.pdf; Marsh Mission, Gary, Rhea and Lockwood CC, “A Week of Exictement, September 25, 2004, Rhea’s Journal 19,” Mission,http://www.marshmission.com/RG_19.htm; McGee, David, P.E., “Geothermal Resources in Louisiana,” Louisiana Department of Natural Resources, Technology Assessment Division, Energy Facts Newsletter, May 2006, http://dnr.louisiana.gov/sec/execdiv/techasmt/newsletters/2006/2006-05_topic.pdf.