Louisiana is losing its wetlands at an approximate rate of 100 km2 per year (Boesch et al. 1983). Although both natural and cultural reasons can be accounted for some of the loss, the greatest amount can be attributed to the construction of canals. Canals have been built for access and transport of oil, which has been extracted from the coastal area. One restoration technique being utilized is backfilling of canals once they have been abandoned by the oil companies. The process of backfilling has not only increased the wetland vegetation, but has also been responsible for significant advances in restoring wildlife habitat and reducing soil erosion. This short-term mitigation process is efficient and economically feasible.

  Louisiana is on the southeastern coast of the United States bordering the Gulf of Mexico. The coastal land is primarily marsh, claiming thirty percent of the nation's coastal wetlands (Craig et al. 1980). Three marsh types - fresh water, brackish and saline, are part of the wetland system. The freshwater marsh is the furthest inland and receives freshwater runoff from the Mississippi River. The saline marsh is situated closest to the sea, and the brackish marsh interfaces the fresh and saline waters. Freshwater marsh vegetation has an extremely low tolerance to saline (sodium < 5 ppt). Seawater has a saline concentration of approximately 20 ppt (Pezeshki et al. 1995). Marshes are relatively shallow, with average depth of less than 2 meters.

In order to understand the effects of canal building in the Louisiana coastal wetlands, a better understanding of land gains and losses is needed. This discussion will help to show the relationship between the wetland systems and the magnitude of loss attributable to canal building. Finally, this paper will discuss alternative approaches to mitigate wetland loss, and how backfilling of canals can reduce further loss and restore previously damaged wetlands.

  Two ways in which land is added to the Louisiana coastline are deposition and accretion. Deposition occurs when sediment from the Mississippi River is deposited into the Gulf of Mexico. Continuous sediment loading will create a delta. As the gradient of the river channel changes, the river will change course and build a new delta lobe. Accretion involves the entrapment of sediment and organic material (peat) into the existing marsh. When the Mississippi River floods, overland flow deposits a rich load of sediment and nutrients onto the marsh. The marsh increases in height, stimulating vegetation growth, creating dense peat formation and further trapping the sediment. Major storm events add an even greater amount of nutrient-laden ocean sediment to the marsh. Accretion increases the height of the marsh by an average of .6 cm to .8 cm per year (Delaune et al. 1996). The shoreline is continuously battered by hurricanes and tidal motion. The saline marsh acts as a physical and chemical buffer, protecting the inland from erosion and invasion of salt water inland.

Land is lost by storms, erosion, saltwater intrusion (invasion), and subsidence. When the Mississippi River changes course, it creates a new delta lobe. The older lobes, no longer receiving sediment deposition, begin to lose mass from the constant battering of storms and tides. Tidal inflow and outflow, through small tributaries and inlets, cause an increase in water velocity. The increase in velocity undercuts the banks through erosion, widening the stream channel. When the channels widen, land mass is removed and saltwater intrudes into saline intolerant freshwater marsh. Subsidence is caused from geologic faulting and folding, a rising sea level and from soil compaction. As a result of subsidence, the sea is moving inward, causing saline water to encroach further inland. Subsidence has caused a slow, but measurable, loss of freshwater marsh.

  Land gains and losses become more complex when humans begin to alter the landscape. Land is gained by filling in marshes for agriculture, development and road construction. Land is also sustained by building levies which prevent storm and tidal erosion.

Land loss has occurred by damming the Mississippi River to prevent it from changing course. No new delta lobes are created. The levies, built for coastal storm protection, also prevent flooding from the river, which reduces sediment deposition through overland flow. Subsidence occurs as a result of ground water, oil and gas extraction. Subsidence is also a product of soil compaction from road construction. Marshland is impounded for agriculture and aqua culture. Impoundment is the process of flooding shallow freshwater marshes, destroying natural vegetation and creating open ponds. Finally, canals are dredged for oil and gas extraction and exploration.

  The primary cause of wetland destruction in the Louisiana marshes comes from the construction of canals. Some startling figures show that between 40% and 90% of the total land loss is directly or indirectly related to canal building (Craig et al. 1987). This practice began in the 1920's and 1930's when exploration and extraction of oil and natural gas began. Canals are dredged to a depth of 2.5 meters and range from 100 meters to 1000 meters in length, some extending from the Gulf of Mexico to the entire length of the coastal shoreline (Turner et al. 1994). They are built because natural channels are not deep enough, nor are they located conveniently for industry requirements. The Louisiana delta area is now laced with thousands of these interconnecting channels.

When the canals are built, the dredged material is thrown up along the side, creating what is called a spoil bank. Spoil banks consist of marsh soil and organic material. As the banks settle, they create a levy that runs along the length of the canal. Estimates show that for every mile of canal built, 30 to 40 acres of marsh are degraded or buried under the spoil banks (Connor et al. 1987). Spoil banks disrupt the natural source of sediments needed for accretion and block overland flow. Although revegetation does occur, species composition changes, creating shrubs and small trees as a result of the higher elevation of the spoil banks (Craig et al. 1980).

The levies created by the spoil banks not only alter the natural hydrological flow, but can also block migration of aquatic organisms. Studies have also shown that certain aquatic species migrate up and inhabit back marshes during their juvenile state, then return to the sea for their adult life (Reed et al. 1994). Because the canals are deeper than the surrounding wetlands, they allow larger predators to enter areas inhabited by juvenile fish. Marked declines in fish catch by some Louisiana commercial fishing industries have been noted as a direct result of predation and loss of migratorial passageways.

Tides enter and leave through the canals at a greater velocity than through undisturbed marsh, resulting in erosion and widening of the banks. Annual increases in canal widening range between 2 to 14% (Craig et al. 1980). Saline water travels up the canals, invading deep into freshwater territory, causing vegetation to die and creating open water where dense vegetation previously existed. The greatest impact on wetland destruction has been noted in saline and brackish marshes. Besides the deepening and widening of the canals through tidal erosion, these marshes are hit the hardest by coastal storms, making them less effective as storm buffers for the inland coast.

Where nutrient-laden sediments would naturally trickle slowly through the wetlands, canals reroute it to lakes and ponds, causing an increase in eutrophication. Canals are, therefore, directly linked to loss of wildlife habitat, and to the decrease in the effectiveness of the marsh as a natural water purifying system (Craig et al. 1980).

  In order to reduce the destructive effects of past and future canal construction, recommendations have been considered and some have been attempted. The following is a discussion of five of these mitigation techniques.

Controlled Diversion: One proposed mitigation techniques was to allow the U.S. Army Corp of Engineers to divert some of the Mississippi River into a controlled area, creating a new delta (Craig et al. 1987). This diversion could create more wildlife habitat and could potentially improve fisheries. This would require prior planning, operational experience and ongoing management. Although the cost and efficiency would have been low, there was a great deal of concern regarding the concentrations of heavy metals and pesticides in both the water and sediment. This potential project was put on hold pending further investigation.

Reuse of Spoil: A 5-year research project was conducted by the U.S. Army Corp of Engineers to reuse material from spoil banks. Some suggestions included deposition as a substrate for wildlife habitat, beach renewal, restoration of bare ground, road construction material and sanitary land filling. The study concluded that spoil material used as a biologically productive substance could be effective and the results could be accurately measured (Craig et al. 1987). Although this alternative could be used as a post-remedial action, it would not halt the ongoing loss from continued dredging and canal building.

Regulatory Controls: Regulatory controls have the advantage of affecting the entire coastal zone area. It requires builders comply with standardized practices. Controls suggested include (1) standardizing the depth of canals and requiring companies to backfill them once they were no longer in use, (2) prohibiting new canal construction, (3) minimizing new canal construction by the use of existing canals, (4) plugging pipeline canals with earthen or shell dams (plugs) wherever possible, (5) no new wetland impoundments, (6) avoiding or eliminating residential development on wetlands, and (7) reserving spoil material to build new marshes. While these methods would not eliminate past damage, but further wetland destruction would be halted.

Saltwater Intrusion Remediation: Remediation efforts have been suggested to reduce the impact of saltwater intrusion. Impoundment or semi-impoundment by constructing low levies with spoil material close to the marsh edge to limit water exchange. Little is known about lthis technique's long term effectiveness. Impoundments do not allow for fish migration. Studies have also shown that hurricanes have a more devastating effect on impounded areas than on natural marshes. A second suggestion was to divert freshwater into the marshes to reduce salinity. A third option to restore the natural hydrology of the wetland system was to permanently close some canals, and to install locks in navigation channels.

Backfilling Canals: The most important of the five mitigation techniques was backfilling of the canals. In one study, 33 canals that had been abandoned by the oil industry were backfilled. The backfilling process consisted of bulldozing spoil banks back into the canals. Removing spoil from banks to encourage revegetation, and restoring the canal depth to its natural state. After backfilling was completed, return visits were routinely done. Sediment deposition and loss was measured, as was canal depth and width (Turner et al. 1994).

The cost of this backfilling project was in the range of $1,200 to $3,400 per hectare. In contrast, building of the original canals were constructed at an average cost of $25,000. Other positive aspects of this technique were that costly equipment was not required and there was no further on-site management required. Once backfilling was completed, the area was left for natural processes to take their course.

Some factors considered for the backfilling project were (1) soils, (2) vegetative cover, and (3) fish utilization. Fish utilization related to canal length, canal age, marsh soil organic matter content, and the presence or absence of a plug at the mouth of the canal (Turner et al. 1994).

The results of backfilling have been effective. Although some canal delineation is still discernible, the edges of the spoil banks are becoming more irregular. Revegetation is occurring on spoil banks, although if the elevation was too high, there was a change in species. Dendritic drainage patterns have reestablished and have begun to revert back to their natural hydrologic flow, especially in areas where no plug was in evidence. Where plugs were intact, wildlife and fish habitats were created but did not allow for migratory aquatic species. In addition, deposition of sediments and nutrients were not apparent.

Some of the most significant factors that influenced the success of the backfilling project was the canal length and whether or not sufficient amount of spoil material was returned to the canal. These factors were directly related to the experience of the dredge operator. Putting the correct amount of spoil material back in the canal without gouging the marsh or causing the marsh elevation to be too high or too low, was a key factor. Because there are so few skilled operators, it was difficult to get consistent results.

Backfilling as a restoration technique to manage abandoned canals is easy, cost effective and does not require further on-site management. The area becomes revegetated, and erosion is halted. The natural hydrology is restored and fish and wildlife become more abundant. As more abandoned canal sites become available, more opportunities will be available for future wetland restoration.

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