Pattern and process of land loss in the Mississippi Delta: A spatial and temporal analysis of wetland habitat change

An earlier investigation (Turner 1997) concluded that most of the coastal w etland loss in Louisiana was caused by the effects of canal dredging, that loss was near zero in the absence of canals, and that land loss had decreas ed to near zero by the late 1990s. This analysis was based on a 15-min quad rangle (approximately 68,000 ha) scale that is too large to isolate process es responsible for small-scale wetland loss and too small to capture those responsible for large-scale loss. We conducted a further evaluation of the relationship between direct loss due to canal dredging and all other loss f rom 1933-1990 using a spatial scale of 4,100 ha that accurately captures lo cal land-loss processes. Regressions of other wetland loss on canal area (i .e., direct loss) for the Birdfoot, Terrebonne, and Calcasieu basins were n ot significant. Positive relationships were found for the Breton (r(2) = 0. 675), Barataria (r(2) = 0.47), and Mermentau (r(2) = 0.35) basins, indicati ng that the extent of canals is significantly related to wetland loss in th ese basins. A significant negative relationship (r(2) = 0.36) was found for the Atchafalaya coastal basin which had statistically lower loss rates tha n the other basins as a whole. The Atchafalaya area receives direct inflow of about one third of the Mississippi discharge. When the data were combine d for all basins, 9.2% of the variation in other wetland loss was attributa ble to canals. All significant regressions intercepted the y-axis at positi ve loss values indicating that some loss occurred in the absence of canals. Wetland loss did not differ significantly from the coast inland or between marsh type. We agree with Turner that canals are an important agent in cau sing wetland loss in coastal Louisiana, but strongly disagree that they are responsible for the vast majority of this loss. We conclude that wetland l oss in the Mississippi delta is an ongoing complex process involving severa l interacting factors and that efforts to create and restore Louisiana's co astal wetlands must emphasize riverine inputs of freshwater and sediments.