Before April 20 when the Deephorizon oil rig exploded, killed eleven oil, workers, and spewed oil into the Gulf of Mexico for three months until BP finally managed to cap the well, I was beginning to write a series of posts on the state of the forests along the Mississippi from the headwaters to the Louisiana Coast. How we have changed and managed the Mississippi has changed how we manage the forests in its floodplain.
Lac des Allemands Watershed: Bayou Chevreuil
St. James Parish, Louisiana
“Bayou Chevreuil extends from near Donaldsonville to Lake Des Allemands, although in places called by other names, and flows along the lowest levels between the Mississippi and the La Fourche. It is the principal drainage outlet of that part of the State, or would be if opened.”–Captain W.L. Fisk, Corps of Engineers, 1891[iii]
Is Louisiana the productivity of bottomland forests, which lie in the northern part of the basin, depends on a reliable wet-dry cycle. The rains come to the Mississippi delta’s hardwoods in the late winter and early spring. The rest of the year the forests are drier with the right amount of moisture and nutrients to encourage healthy growth. Bottomland hardwoods grow best when nature follows this regime. Change the regime, the trees become stressed, and their productivity falters. With subsidence, water levels rise, the forest becomes wetter, the hardwoods–green ash, bitter pecan, and Nuttal oak–disappear to be replaced by cypress and tupelo.[i]
The forest zones in the Louisiana coastal region are similar to those in Mississippi and Arkansas. The cypress and tupelo of the swamps are adapted to continuous flooding, though the cypress will only germinate on moist mud flats, not in standing water. Mixed in with the cypress can be black willow, water elm, water ash, and buttonbush. Water hickory, red maple, green ash, and river birch occupy the slightly drier land of the lower hardwood swamp forest. Royal fern, jewelweed, and butterweed cover the understory. The flats, where sweetgum, sycamore, laurel oak, and willow oak form the canopy, are only seasonally saturated. Woody vines–poison ivy, greenbriers, and trumpet creeper–wrap the trunks.
When the bottomland hardwood and cypress swamps of the northern Barataria basin flooded naturally, they served as storage areas, captured sediment, and soaked up nutrients. Their broadleaf deciduous trees contributed an annual load of litter, which decayed into organic sediment and counteracted natural rates of subsidence. As Captain Fisk noted, Bayou Chevreuil carried the excess water from the swamps to Lac des Allemands.
Several projects altered the natural flow of water west to east through the swamps northwest of Lac des Allemands and created an impoundment. On the west engineers ran Louisiana Highway 20 on a small embankment north through the swamps in 1930. Vacherie Canal was dredged from Highway 20 past the Golden Star Plantation to Lac des Allemands in 1955, creating a spoil bank on the south. The Corps of Engineers dredged and straightened Bayou Chevreuil for flood control in 1959, creating a second spoil bank on the north. Connect the spoil banks to a natural levee on the southeast and 2,397 acres of swamp were impounded and flooded in water over three feet deep. The swamps declined.
What has happened to the 242,000 acres of bottomland forests and wooded swamps in the Barataria basin is a paradox. The construction of the Mississippi River levees, not more than ten miles to the north of this forest, disrupted the seasonal pattern of flooding and drying and deprived the forests of freshwater, sediment, and nutrients. Without the sediment to counteract subsidence and with water levels rising in southern Louisiana at the rate of 3.3 feet per century, Barataria’s forests became flooded almost year-round.
Subsidence in the upper Barataria basin, the Lac des Allemands watershed, led to flooding and a thirty-eight percent decrease in bottomland hardwood forest between 1972 and 1992. With flooding some of forest converted to swamp, treed in cypress and tupelo; much of it converted to marsh. Urbanization claimed some of the forest. Should the loss continue at such a rate, the bottomland hardwoods of the upper Barataria basin would disappear by 2025.[ii]
[i] Conner, William J., Day, John W, and Slater, Wayne R., “Bottomland Hardwood Productivity: Case Study in a Rapidly Sudsiding Louisiana, U.S.A., Watershed,” Wetlands Ecology and Management, Vol. 2, No. 4, 1993, 189.
[ii] Coastal Wetland Forest Conservation and Use Science Working Group, Conservation, Protections and Utilization of Louisiana’s Coastal Wetland Forests, Final Report to the Governor, April 30, 2005, 37-40; Nelson, S.A., Soranno, P.A., and Qi, J, “Land-cover Change in Upper Barataria Basin estuary, Louisiana, 1972-1992: Increases in Wetland Area,” Environ Manage, May 2002, 716-27, http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?CMD=Display&DB=pubmed.
[iii] The Executive Documents of the House of Representatives , 1st Session, 52nd Congress, 1891-1892, Washington, D.C., U.S. Government Printing Office, 1892, 1841.
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