Sediment infilling and wetland formation dynamics in an active crevasse splay of the Mississippi River delta Donald R. Cahoon a, ⁎ ,1 , David A. White b , James C. Lynch a,1 a United States Geological Survey, National Wetlands Research Center, 700 Cajundome Boulevard, Lafayette, LA 70503, USA b Department of Biological Sciences, Loyola University, New Orleans, LA 70118, USA abstract article info Article history: Received 15 April 2010 Received in revised form 30 November 2010 Accepted 2 December 2010 Available online 13 December 2010 Keywords: Crevasse splay Mississippi River delta Wetlands Accretion Shallow subsidence Elevation Crevasse splay environments provide a mesocosm for evaluating wetland formation and maintenance processes on a decadal time scale. Site elevation, water levels, vertical accretion, elevation change, shallow subsidence, and plant biomass were measured at five habitats along an elevation gradient to evaluate wetland formation and development in Brant Pass Splay; an active crevasse splay of the Balize delta of the Mississippi River. The processes of vertical development (vertical accretion, elevation change, and shallow subsidence) were measured with the surface elevation table–marker horizon method. There were three distinct stages to the accrual of elevation capital and wetland formation in the splay: sediment infilling, vegetative colonization, and development of a mature wetland community. Accretion, elevation gain, and shallow subsidence all decreased by an order of magnitude from the open water (lowest elevation) to the forest (highest elevation) habitats. Vegetative colonization occurred within the first growing season following emergence of the mud surface. An explosively high rate of below-ground production quickly stabilized the loosely consolidated sub- aerial sediments. After emergent vegetation colonization, vertical development slowed and maintenance of marsh elevation was driven both by sediment trapping by the vegetation and accumulation of plant organic matter in the soil. Continued vertical development and survival of the marsh then depended on the health and productivity of the plant community. The process of delta wetland formation is both complex and nonlinear. Determining the dynamics of wetland formation will help in understanding the processes driving the past building of the delta and in developing models for restoring degraded wetlands in the Mississippi River delta and other deltas around the world. Published by Elsevier B.V. 1. Introduction The 32,000 km 2 Mississippi River delta consists of shallow estuaries, wetlands, and distributary ridges that formed from six overlapping delta lobes during the past 6000 years (Coleman et al., 1998). This area is an ecologic and economic engine for the Gulf of Mexico region and the United States as a whole (Twilley, 2007). The vast estuarine and wetland area provides 30% of the US total fish catch and is a critical stopover for neotropical migrating birds and waterfowl in the Central Flyway. The wetlands store water, filter sediments and pollutants from the water, help stabilize shorelines, and protect human settlements by ameliorating storm surges associated with hurricanes. There is extensive human settlement across the delta, including commercial ports that handle more than 20% of the nation's foreign waterborne commerce. However, during the past few centuries, human use of the Mississippi River watershed and its delta ecosystem has dramatically altered the delta, causing a shift to the destructive phase of the delta cycle (Coleman et al., 2008; Blum and Roberts, 2009) resulting in extensive wetland loss and a growing demand to restore the delta environment (Day et al., 2007). Multiple factors are contributing to the deterioration of the delta. Dams on the Mississippi River and its tributaries have reduced the sediment load in the River by 48% (Syvitski et al., 2009; Blum and Roberts, 2009) and flood protection levees prevent annual spring flooding of the delta plain and most of the sediment entrained in the river from reaching the delta plain wetlands. Construction of an extensive network of canals for oil and gas exploration and navigation has altered local hydrology within delta plain wetlands that has contributed to the impounding of water and related stresses to the marsh vegetation (Day et al., 2000; Ko and Day, 2004). In addition, extensive sub-surface fluid extraction, particularly oil and gas, has led to accelerated subsidence from reservoir compaction and fault reactivation across the delta plain (Morton and Bernier, 2010; Morton et al., 2006). The synergistic effect of reduced sediment load, altered local marsh hydrology, and accelerated subsidence has resulted in an Geomorphology 131 (2011) 57–68 ⁎ Corresponding author. Tel.: +1 301 497 5523; fax: +1 301 497 5624. E-mail addresses: dcahoon@usgs.gov (D.R. Cahoon), dawhite@loyno.edu (D.A. White), jclynch@usgs.gov (J.C. Lynch). 1 Present Address: United States Geological Survey, Patuxent Wildlife Research Center, c/o BARC-East, Building 308, 10300 Baltimore Avenue, Beltsville, MD 20705, USA. 0169-555X/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.geomorph.2010.12.002 Contents lists available at ScienceDirect Geomorphology journal homepage: www.elsevier.com/locate/geomorph