St. Petersburg Coastal and Marine Science Center
Historical Wetland Loss
Analysis of historical satellite imagery and previously interpreted land-classification datasets identified more than 4,600 km2 of low-lying wetlands in coastal Louisiana that converted to open water between 1932 and 2004. Land-loss curves generated from these datasets showed that peak land-loss rates in coastal Louisiana exceeded 100 km2/yr between 1956 and 1978. However, these rates were averaged over multi-decadal (10- to 20-year) time periods and lacked the temporal resolution to more precisely constrain short-term land-area changes. In comparison, visual analysis of historical aerial photography at several delta-plain sites suggested that much of the interior wetland losses occurred within a much shorter time period. In collaboration with John Barras (USGS – Eastern Geographic Science Center), SPCMSC scientists analyzed aerial photography from 1969 and 1974 encompassing 670 km2 at five representative delta-plain wetland-loss hotspots (Madison Bay, Pointe au Chien, Bully Camp, DeLarge, and Bay St. Elaine study areas) in an effort to further constrain the timing and duration of peak wetland loss. The resulting land-loss curves showed that the most rapid wetland loss at these study areas occurred during the late 1960s and 1970s. Peak wetland-loss rates during this period were two to four times greater than both the pre-1970s background rates and the most recent wetland-loss rates.
Compared with the upper delta-plain interdistributary study areas (Madison Bay, Pointe au Chien, Bully Camp, and DeLarge), where extensive areas of formerly emergent marsh rapidly converted to open water before 1978 with little significant land-area change since, visual analysis of historical aerial photography at the other study areas indicates that large-scale historical wetland losses at most of the lower delta-plain sites persisted until about 1990. At Leeville, Fourchon, and Caminada study areas in the lower delta plain, some wetland loss had initiated by the mid to late 1970s, but land-loss rates accelerated after 1978, and the majority of historical land loss occurred between 1978 and 1990. Especially at Leeville and Fourchon, some areas of formerly continuous marsh were alternately emergent or partially submerged on historical aerial photographs acquired prior to 1978 depending on whether the images were acquired under low or high water conditions, respectively. Between 1978 and 1990, most of these “wet marsh” areas became permanently submerged. At Sabine National Wildlife Refuge (SNWR) in the western chenier plain, most of the historical wetland loss occurred prior to 1978; however, some expanses of wet marsh surrounding Greens Lake in western SNWR persisted into the 1980s.