Dredged Canals, Wetland Loss, and Legacy

Abstract

The direct effects of converting coastal wetlands to open water by dredging them can be magnified by indirect effects. For example, dredged canals allow for recovery of mineral fluids 1000 s of m belowground which may induce geological subsidence or faulting; the dredged material deposited at the surface creates levees that redirect overland water flows. These indirect factors may stress wetland plants enough so that additional wetland habitat is converted to open water as a result of longer intervals of wetland soil waterlogging and drying, sulfide toxicity, less organic matter and sediment accumulation, and greater erosion. We quantified the indirect effects by demonstrating a robust dose–response relationship between coastal land loss and canal density in the Mississippi and Niger river deltas over 5 decades. Importantly, the ratio of land loss to canal area increases with time—a legacy effect. Surface impediments to water movements rather than belowground subsidence are the dominant causal factor. We also found that flood protection levees on the main river channel did not significantly magnify the effect of dredging on wetland loss. The cumulative effect of these direct and indirect consequences in coastal Louisiana is enormous and continuing, equaling many tens of billions dollars annually. Understanding these effects supports the rejection of a hypothesis that regional river channel flood protection levees or fluid withdrawal is of greater importance than the local changes in wetland hydrology. Wetland restoration/mitigation of dredging impacts on these two coasts can be implemented at a relatively low cost and quickly if this paradigm of the causes of coastal wetland losses is adopted.