Gulf Coast Wetland Shoreline Change

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Land and seafloor slopes are generally low along the coasts in the Mid-Atlantic and Gulf Coast states, making wetlands and estuaries vulnerable to sea level change, subsidence, and extreme events (e.g., hurricanes and tropical storms). Land-use change and land loss have been mapped extensively and with increasing frequency, but the link between land loss and the processes responsible for landscape change has not been investigated in detail in many places. 

Image: Wetland Impacts from Hurricane Isaac

Photo of Upper Breton Sound, Louisiana, after Hurricane Isaac, showing how the storm removed vegetation and where storm surge deposited extensive wrack (organic debris and trash) and created or expanded channels. Credit: Brady Couvillion, USGS

Aerial imagery sequence of Indian Point, Louisiana, showing shoreline erosion since 1950

Aerial imagery sequence of Indian Point, Louisiana, showing the contraction of the peninsula marsh in response to shoreline erosion since 1950. Credit: USGS

CMHRP scientists study how coastal wetlands and estuaries interact on scales from years to hundreds of years. One such CMHRP effort was in Mobile Bay, Alabama. It was initiated as part of the National Gulf of Mexico project and has continued as part of the Sea-level and Storm Impacts on Estuarine Environments and Shorelines (SSIEES) project. Findings show that natural disturbances such as hurricanes strongly affect the estuary and marsh. The bayhead delta and the marshes fringing the bay were found to rely heavily on inorganic sediment flux from the adjacent river channels and the estuary, respectively. In the fringing marshes, sediment properties, stable isotopes, and timelines derived from lead and cesium isotopes indicated that hurricanes and storm surge were the main processes delivering inorganic sediment to the marshes. Using modeled marsh elevation, the CMHRP showed that without that event sedimentation, the marsh elevation might have fallen below mean sea level.

 

Venn diagram highlighting important controls on living shorelines such as marsh wetlands and mangrove forests

Venn diagram highlighting important controls on living shorelines such as marsh wetlands and mangrove forests. CMHRP scientists focus on the interactions among biogeochemical, geological, and physical variables, and through collaborative efforts with other USGS Mission Areas, other agencies and bureaus, as well as academia, they address interactions with species and ecosystems. Credit: USGS

The findings from Mobile Bay highlight some of the beneficial aspects of sediment deposition during storms; however, large portions of shorelines can also be lost during storms and fair-weather conditions. To learn more about those processes, SSIEES studies have expanded into Louisiana, Mississippi, and additional regions of the Alabama shoreline (Mississippi Sound) and the surrounding bays and estuaries, as well as Chincoteague Bay (Maryland and Virginia) and Barnegat Bay (New Jersey). The efforts have integrated a wide selection of proxies (microfossils, stable isotopes, biomarkers, metals, and other sediment tracers) to better quantify marsh response laterally and vertically to storms and sea level, as well as the interactions between the two. The CMHRP partners for these efforts with the USGS Ecosystems Mission Area (National Wetlands Research Center -Lafayette), Grand Bay National Estuarine Research Reserve, the U.S. Fish and Wildlife Service through Grand Bay National Wildlife Refuge and Bon Secour National Wildlife Refuge, the National Park Service (National Seashore and Assateague Island), Chincoteague Bay National Wildlife Refuge, Maryland and Virginia state wildlife agencies, and academic colleagues at several universities.

Photo showing complex geomorphology of the Grand Bay marsh landscape

Photo showing the complex geomorphology of the marsh landscape of the Grand Bay National Wildlife Refuge/Grand Bay National Estuarine Research Reserve in coastal Alabama and Mississippi. (1) Geology—a tidal creek that at lower sea level than present served as a distributary channel of a river-delta system. (2) Hydrodynamics—wave erosion of the marsh edge. (3) Geochemistry/biogeochemistry—rich brown sediment exposed on the escarpment that either ends up back on the marsh or in the estuary. (4) Biology—patches of vegetation (bright green vs. dull brown) adapted to slightly different environmental conditions. Credit: Terrence McCloskey, USGS

"Sea Level Rise, Subsidence, and Wetland Loss" video screenshot

Sea Level Rise, Subsidence, and Wetland Loss: This video describes causes of wetland loss in the Mississippi River Delta. Rapid land subsidence due to sediment compaction and dewatering increases the rate of submergence in this deltaic system. The construction of levees along the lower Mississippi River also has reduced delivery of sediments to coastal wetlands, which have been deteriorating as soil surfaces sink and wetland plants are subjected to excessive flooding. Other factors that have contributed to land loss include construction of canals and periodic hurricanes. Sea level rise can lead to movement of saltwater inland, but coastal plants tolerate salinity through several morphological and physiological mechanisms. The causes of wetland loss are complex and not the result of any single factor. Natural and anthropogenic factors have combined with global processes such as sea level rise to cause wetland loss in the Mississippi River Delta. Credit: Karen McKee, USGS.