ABSTRACT

Turner, R.E., 2014. Discussion of: Olea, R.A. and Coleman, J.L., Jr., 2014. A synoptic examination of causes of land loss in southern Louisiana as related to the exploitation of subsurface geological resources. Journal of Coastal Research, 30(5), 1025–1044.

I comment on Olea and Coleman's (2014) conclusion that subsidence was the primary cause of the dramatic rise in Louisiana's coastal land losses in the last 100 years. The focus on subsidence combined with the omission of context for factors not related to subsidence (e.g., dredged canals), leaves the reader with the incorrect conclusion that anthropogenic factors observed to date are insignificant, and that coastal wetland losses are only driven by subsidence. I address this omission by discussing two points about anthropogenic influences: (1) dredged canals and (2) changes in sediment load from the watershed and its distribution. They omit quantitative inclusion of two signature symptoms of the cause-and-effect relationships at temporal and spatial scales. To whit, there are: direct relationships between canal density and land loss over decades and shorter intervals for the whole coast and individual estuaries, instances of indirect losses immediately after canal construction, an increase in ponding near dredged canals but not further away, and, evidence of effective hydrologic barriers created by the spoil bank above- and belowground. The view that geological subsidence exerts a top-down control on the net adjustment to changes in vertical space leads to the narrow view of restoration being modeled using the mineral soils for wetland soils comprised mostly of organics. Further, the decline in suspended sediment concentrations since the 1950s (from dam construction) needs to be put within the context of the landscape changes occurring when European colonization resulted in much higher rates of erosion. The restriction of exclusively geological factors driving land loss is, therefore, an incomplete view of what causes land loss in modern times—and a perhaps dangerously naïve basis for management decisions on this coast. I agree with their conclusions that (1) geological subsidence has not changed significantly in the last 100 years, (2) fluid withdrawal is an unlikely and unproven large enough force to cause the patterns in land loss across the deltaic plain, and (3) acceleration in sea level rise will rise to problematic levels in the near future.

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