THE GEOMORPHOLOGY AND PROCESSES OF LAND LOSS IN THE PONTCHARTRAIN BASIN, SOUTHEAST LOUISIANA: 1990 – 2001

The Pontchartrain Basin is a 12,173 km2 watershed in southeastern Louisiana that is losing land due to a complex suite of causes. Government agencies and the oil and gas industry have been targeted as the primary agents of coastal land loss; however the role of natural processes and the multiple causality of the coastal land loss problem have often been overlooked. The objective of this presentation is to present the results of a project sponsored by the Pontchartrain Restoration Program to map and quantify the geomorphology and processes of land loss in the Pontchartrain Basin for the time period 1990-2001.

Results of the land loss classification indicate that between 1990 and 2001, 136.25km2 of land was converted to open water in the Pontchartrain Basin, which equals a land loss rate of 12.39km2/yr. The geomorphology of the loss was primarily shoreline loss, which accounted for 87.76%, leaving interior loss to account for 12.24%. The process driving the loss was primarily erosion, which accounted for 87.76% of the loss, followed by submergence, which accounted for 11.54% of the loss and direct removal, which accounted for only 0.70% of the loss.

Three areas of concentrated land loss are visible within the basin and include: (1) where the Mississippi River Gulf Outlet (MRGO) intersects with the Inner Harbor Navigation Canal (IHNC) and the Intracoastal Waterway and along the perimeter of Lake Borgne, (2) just south of the Caernarvon Fresh Water Diversion near Lake Leary, and (3) at Baptiste Colette near the mouth of the river. The majority of the land loss occurring in area (1) is the result of erosional processes along both the Lake Borgne shoreline and along the shorelines of an extensive network of canals that were built in the area prior to 1990. The canals contribute to salt water intrusion, which intensifies erosion by causing the death of the native marsh plants exposing the soil to the erosive action of water. Both shoreline and interior land loss through the processes of erosion and submergence are occurring in area (2). This brackish habitat is rapidly disintegrating as the shorelines of ponds and lakes that formed prior to 1990 erode and new ponds emerge as the land surface subsides. The process of loss in area (3) is primarily erosion and the geomorphology of the loss is primarily shoreline loss along the edges of the marsh and channel.