QUANTIFYING HISTORICAL ACCOMMODATION FORMATION, SABINE NATIONAL WILDLIFE REFUGE, SOUTHWEST LOUISIANA

A prior U.S. Geological Survey study documented large volumes of new accommodation that formed at 10 study areas within the Mississippi River delta plain since the mid-1950s in association with widespread, rapid land-surface subsidence and conversion of coastal wetlands to open water. This study extends that work, integrating data from remotely sensed images, sediment cores, and water-depth surveys to assess historical accommodation formation at five additional wetland sites in Sabine National Wildlife Refuge (SNWR) in the western chenier plain.

The 1-D (vertical) processes responsible for accommodation formation were quantified by comparing surface elevations, water depths, and vertical displacements of stratigraphic contacts that were correlated between short sediment cores. Results of these analyses indicate that erosion contributed more than subsidence to the formation of accommodation associated with historical wetland loss at SNWR. Integration of data from remotely sensed imagery and water-depth surveys at each of the study areas provided a basis for estimating the total 3-D accommodation (volume) formed by land-surface subsidence and subsequent erosion. At the five study areas, more than 15 x 10⁶ m³ of accommodation formed across areas of formerly continuous interior marsh. In total, more than 65 x 10⁶ m³ of new accommodation formed within SNWR between 1956 and 2004. These volumes provide estimates of the new sediment that would be needed to restore the SNWR wetlands to their pre-1956 areal extent and elevations and represent just a fraction of the total accommodation associated with western chenier-plain wetland loss since 1956.

Despite their different geological settings, the temporal and spatial trends of historical wetland loss are similar in both the western chenier and delta plains. Analysis of historical imagery identified expanses of wet marsh (the marsh surface is submerged but still visible, representing nearly uniform drowning of large sections of marsh), indicating that land-surface subsidence was the process that initiated historical wetland loss and accommodation formation in both regions. Compared with the delta plain, however, magnitudes of subsidence, 1-D accommodation, and 3-D accommodation were generally less at the SNWR study areas.