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Support River Reintroduction into Maurepas Swamp

November 11, 2014 12:09 PM

Added November 14, 2014: Letter of support signed by local organizations and community leaders for the River Reintroduction Into Maurepas Swamp project.  This diversion is on the list of the 5 projects selected by CPRA for Louisiana to submit to the RESTORE Council for "bucket" 2 funding consideration. Take action via La Camo Coalition.  View the Project Fact Sheet and Project plans

 

The River Reintroduction into Maurepas Swamp project, also known as the Maurepas diversion or the West Maurepas Diversion (and associated with Hope Canal early in its history), is designed to restore systems and processes that served the swamp before levees restrained the Mississippi River. The project has a long history in coastal restoration planning, starting with the The Louisiana Coastal Restoration Plan in 1993, the Louisiana Coast 2050 report in 1998, the Mississippi River Sediment, Nutrient and Freshwater Diversion Study in 1999, and both Coastal Master Plans in 2007 and 2012.

As of October 2014, the Coastal Protection and Restoration Authority (CPRA) is considering it as one of their five projects to nominate to the RESTORE Council (Council) for RESTORE Act “bucket 2” money. Each council member can propose five projects for consideration by November 17th. Bucket 2 is dedicated for ecosystem restoration, and is the only bucket of RESTORE Act money (of which there are five), that the Council itself will decide how the money is spent.

The Maurepas area includes Lake Maurepas and surrounding swamp and marsh. The lake is connected at the south end to Lake Pontchartrain through Manchac Pass. It is fed by several freshwater sources, including the Amite, Tickfaw and Blind Rivers. The surrounding swamp is the second largest coastal forest in Louisiana, and encompasses 191,630 acres of swamp and 12,859 acres of marsh. The swamp was once connected to the Mississippi River, but levees have isolated it for several decades. In that time, the swamp has been starved of nutrients, freshwater and sediment that previously were delivered by the Mississippi River. Saltwater intrusion events make their way up from the Gulf of Mexico, through Lake Pontchartrain and eventually into Lake Maurepas and the surrounding swamp.

Studies show that 87% of Maurepas swamp is dying, largely due to nutrient deprivation, saltwater intrusion, and subsidence.  Each of these stressors can be attributed to the swamp’s isolation from the Mississippi River.

(1) Nutrient deprivation

Plants and trees need nutrients and sunlight to produce energy to grow and survive. The effect of nutrient deprivation on tree growth (primary production) can be measured by litter fall and tree growth. In Maurepas swamp, the further trees are from running freshwater, the less they grow. Some studies have tested adding nutrients to vegetation to mimic Mississippi River water delivery, and it had significant impact on vegetative biomass. Adding nutrients to the swamp by means of the Mississippi River water is expected to improve productivity. Trees in the interior of the swamp where salinities are relatively low are dying of nutrient deprivation.

(2) Saltwater intrusion

Saltwater is intruding from the Gulf of Mexico through Lake Pontchartrain, and therefore salinity is highest closest to Pass Manchac. Increased water and soil salinities kill the cypress and tupelo tress. As you move away from from Pass Manchac salinities decrease. Trees in the swamp on the edge of Lake Maurepas are dying of salt stress. Trees can withstand short saltwater intrusion events, and an influx of freshwater helps them recover. Areas that are touched by flowing freshwater have the lowest salinities. Also, primary production is highest at these sites. However, it takes several years after a saltwater intrusion event such as a drought for soil salinities to freshen. The few trees left at parts of the swamp with high salinities are nearly all cypress, which are more tolerant to saltiness than other trees.

(3) Subsidence and Impoundment- subsidence and continuous flooding is another major stress on the Maurepas Swamp.

While cypress and tupelo trees can survive in water, saplings need dry periods to take root and get established. Therefore, recruitment of cypress and tupelo saplings is essentially absent in the Maurepas swamp. There is a lot of organic material in the swamp. Healthy tree and roots systems help maintain soils, and fight subsidence. The highest bulk densities (a measurement of soil weight= dry weight of soil per unit volume of soil) are found in parts of the swamp with consistent freshwater input (low salinity), and lowest bulk densities occur at sites with high salinity. 

The habitat condition map above is from Shaffer et al 2009. The purple is natural marsh, and red is degraded swamp (swamp that has converted to marsh in the past half century). Yellow, or relict swamp, denotes areas that are stagnant and nearly permanently flooded. Light green refers to throughput sites which are sites that receive nonpoint sources of freshwater on a regular bases. Dark green is bottomland forest. 

A freshwater diversion from the Mississippi river can help fight each of these stressors, and maintain or even increase the health of the swamp. The Maurepas diversion is currently planned as a maximum 2000 cubic feet per second (cfs) diversion, employing three 12 x 12 box culverts. The gated structures are designed so that they can be adaptively managed, changing operation based on the needs of the swamp (pulsing, high water events, saltwater intrusion events, etc.). A water conveyance channel will connect the Mississippi River north of Garyville to the Hope Canal. It will have to cross several major infrastructures, including Airline Highway, railroads, and I-10. A sedimentation pond will allow the heavy sands to fall out right after the structure because the water must convey approximately five miles to the outfall area. A pump station north of Airline Highway will help convey the water through the rest of the five mile journey, and lateral discharges will allow some water to maintain the swamp between Airline Highway and I-10. The remainder will maintain swamp between I-10 and Lake Maurepas. The diversion is designed to maximize “sheet flow” (the spread of water over a wide swath of the receiving area, or a “nonpoint” water source) which is important for improving water quality, increasing primary production, and decreasing salinities deep into the swamp. This is as opposed to more “point sources” of freshwater accomplished by “jet flow”, which bypass the swamp and empty directly into Lake Maurepas.

Maurepas Project Area

The above map illustrates the proposed project area. 

The total area expected to benefit from the reconnection to the Mississippi River will be 45,000 acres of wetland. Freshwater will help deliver nutrients to nutrient-starved trees at the interior of the swamp, and lower saltiness killing trees at the margin of Lake Maurepas. The health of the entire basin is expected to benefit from the input of freshwater. Healthy trees help maintain healthy soils, which helps fight subsidence and provides flood and storm protection. Cypress and tupelo swamps are believed to be better storm and flood protection that other wetlands habitats such as brackish or saltwater marsh, and even mangrove forests.  This diversion is also expected to have benefits to the Maurepas Swamp Wildlife Management Area (WMA). Enhanced fish and wildlife habitat can result from increased primary production of trees and plants, and lower water and soil salinities.

The trees are dying, the soils are weakening from increasing salinities, and the area is converting from swamp to marsh. This increases the risk to communities from flood and storm damage. Not only would Maurepas be the state’s first diversion aimed at maintaining swamp habitat, it would also be the first with an adaptive management plan that allows for the operation of the diversion based on the needs of the swamp.

Citations and Additional Resources:

Gary P. Shaffer, William B. Wood, Susanne S. Hoeppner, Thais E. Perkins, Jason Zoller, and Demetra Kandalepas (2009) Degradation of Baldcypress–Water Tupelo Swamp to Marsh and Open Water in Southeastern Louisiana, U.S.A.: An Irreversible Trajectory?. Journal of Coastal Research: Special Issue 54: pp. 152 – 165.

P. A. Keddy, D. Campbell, T. McFalls, G. P. Shaffer, R. Moreau, C. Dranguet, R. Heleniak (2007) The Wetlands of Lakes Pontchartrain and Maurepas: Past, Present and Future. Environmental Reviews: 15: pp. 43-77.

Louisiana Coastal Protection and Restoration Authority: http://lacoast.gov/new/Projects/Info.aspx?num=po-29

State Master Plan, 2012: http://coastal.la.gov/a-common-vision/2012-coastal-master-plan/

Coastal Wetlands Planning, Protection and Restoration Act: http://coastal.la.gov/project/river-reintroduction-into-maurepas-swamp/

Project Fact Sheet: http://lacoast.gov/reports/gpfs/PO-29_hq.pdf 


 
 
 
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