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Examination of Factors Contributing to the Growth and Loss of Wetlands in Louisiana


The main growth factor of present-day southern Louisiana wetlands is the deposition of sediments from Mississippi River delta. Concurrently, three natural forms of subsidence have been leading to wetland loss: (1) downward flexure of the lithosphere from the accumulation of up to 20,000 m of post-Triassic sediments deposited on the northern margin of the Gulf of Mexico sedimentary basin; (2) basinward sliding of this sediment accumulation and associated normal faulting of the sediments; and (3) compaction of this accumulation by simultaneous compression of sediments and expulsion of fluids. Prior to the 1900s, coastal wetlands were being constructed at an average rate of 250 km2 per century. By the 1930s, there was quantitative evidence of reversal in wetland expansion, by which time new basinwide factors of anthropogenic origin had appeared. After the Mississippi River flood of 1927, engineering modifications to control flooding and improve navigation reduced the amount of sediments reaching the coast by eventually channelling their discharge onto the continental platform away from the wetlands. Since about the beginning of the Industrial Revolution there has been an accelerated worldwide sea level rise. Other influences have been: (1) production of sulfur, oil, gas, and groundwater; (2) construction of navigational channels; (3) digging of oil and gas production pipelines; and (4) changes in river water chemistry contributing to soil biodegradation. Since the 1930s, wetland land loss has been at an annual average rate of 61 km2 per year. By using Monte Carlo simulation, we compared actual total subsidence rates against the combined effect of all regional forms of subsidence in coastal Louisiana. In doing so, we clarified the magnitude and relative importance of those processes. Among the anthropogenic causes, our study indicates that subsidence associated with historic oil and gas production has an average contribution of 5% relative to the natural causes of subsidence. Canal construction has been brought to an essential minimum after contributing about 20% of the wetland areal losses. This leaves sediment supply and sea level rise as the factors of main concern for the future. Controlled diversion of Mississippi River sediments into adjacent wetlands may help offset diminished sediment supply and associated subsidence in local areas.