USGS Reports Preliminary Wetland Loss Estimates for Southeastern Louisiana from Hurricanes

Hurricanes Katrina and Rita transformed some 100 square miles of marsh to open water in southeastern Louisiana, according to preliminary estimates by U.S. Geological Survey (USGS) based on an analysis of satellite data from September and October.

Future observations of Landsat satellite imagery over the upcoming year will allow scientists at the USGS National Wetlands Research Center (NWRC) in Baton Rouge and Lafayette, La., to determine how much of the loss is permanent and how the marsh recovers, officials said on Nov. 1. Although this early analysis of wetlands does not take into account some marsh recovery, indications are that much of the loss may be permanent. Some of the new areas of open water will likely become new lakes.

Most of the loss east of the Mississippi River is attributed to the effects of Hurricane Katrina's storm surge, although Hurricane Rita's surge appears to have rearranged some of the wrack, or marsh debris, left behind by Hurricane Katrina in the upper Breton Sound area.

Substantial marsh loss, primarily from Katrina, occurred east of the Mississippi River in St. Bernard and Plaquemines parishes. Approximately 39 square miles of marsh around the upper and central portions of Breton Sound were converted to open water by ripping of the marsh or by marsh submergence. Large compressed marsh features several thousand feet long are evident in Breton Sound. Most of the loss was concentrated in an area bounded by the Mississippi River levee to the west, the Delacroix Ridge to the east, and State Highway 300 to the north. Follow-up imagery and aerial photography will be used to determine if some of the submerged marshes reemerge over time.

An additional 47 square miles of marsh were lost throughout the Pontchartrain, Pearl River, Barataria, and Terrebonne basins. The active Mississippi Delta also incurred approximately 14 square miles of loss. The lower Pearl River basin contains numerous marsh rips south of Highway 90.

Direct impacts from Hurricane Rita were not as severe as Hurricane Katrina's impacts in southeastern Louisiana. For example, rips in marshes from Rita were not nearly the size of rips from Katrina in upper Breton Sound although they are noticeable in the Barataria and Terrebonne basins. Rita's surge caused new tears in fresh and intermediate marshes within Barataria and Terrebonne basins and reactivated older hurricane scars attributable to Hurricane Lili (2002) in western Terrebonne and the East Cote Blanche Bay area.

Rita's surge caused detectable marsh loss west of the Mississippi River to the Texas border that could not be attributable to Katrina based on analysis of satellite imagery obtained a week after Katrina's landfall, but prior to Rita's landfall.

Now that the compounded effects of the storms on southeastern Louisiana have been analyzed, NWRC scientists are analyzing Landsat imagery to quantify Rita's impacts in southwestern Louisiana.

To perform satellite analysis, USGS scientists in Louisiana used remote sensing technologies and geographic information systems. They compared land and water areas identified by using Landsat 5 Thematic Mapper satellite imagery. Landsat data from November 11, 2004 were compared to data acquired on Sept. 7, Sept. 16, Oct. 9, Oct. 18 and Oct. 25 to identify potential wetland loss.

USGS: http://www.usgs.gov

USGS National Wetlands Research Center: http://www.nwrc.usgs.gov

This article originally appeared in the 11/01/2005 issue of Environmental Protection.

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