WETLAND SUBSIDENCE AND ACCRETION IN THE ATCHAFALAYA RIVER BASIN RELATED TO HUMAN DISTURBANCE

Wetland loss is a great concern along coasts where they may provide the most benefit to society but are also most threatened by anthropogenic disturbances. Coastal wetland loss and impairment has been caused in-part by channelization, sub-surface extraction, and hydrologic diversions. The Atchafalaya River Basin is a distributary of the Mississippi River containing the largest contiguously forested riparian wetland in North America. The Basin also produces one of the only growing coastal wetlands along the northern Gulf of Mexico. Water and sediment inputs from the Mississippi R. are determined by a control structure that preferentially transports sediment through the Basin. Internal flow patterns within the Basin are impacted by a series of petroleum access canals that create abnormally high sedimentation (>1 m/yr vertical) in hydrologically well-connected areas and hypoxia and sediment starvation in flow isolated areas. Subsidence from surficial compaction and sub-surface extraction of oil and natural gas has also impacted the extensive wetland complex.

Sedimentation trends were monitored on approximately 20 floodplain transects in areas identified as highly depositional. Sediment elevation tables (SETs) were placed throughout the Basin to measure subsidence. Sites are visited annually following installation in 2010 and 2011. Deposition and subsidence rates, sediment texture, bulk density, and loss on ignition (percent organic material) were determined at each site. Mean gross sedimentation rates ranged from slightly erosional to greater than 1 m/yr. Sites with low elevation, high hydrologic connectivity, and hydraulic damming (flow stagnation) featured the highest amounts of sediment trapping in the Basin; the converse in any of these factors typically diminished sediment trapping. Subsidence ranges from 1 to greater than 3 cm/yr and is generally greater than associated gross accretion rates, even at moderately high deposition sites. The Atchafalaya Basin sequesters substantial amounts of sediment and facilitates delta growth in the northern Gulf. However, the interior Basin is losing forested wetlands to conversion to open water due to hydrologic diversions and subsidence.