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Wetland surface elevation, vertical accretion, and subsidence at three Louisiana Estuaries receiving diverted Mississippi River water

Wetland surface elevation and vertical accretion were measured from 1996 to 1999-–2000 using a sediment elevation table (SET) and feldspar marker horizons in nine paired wetlands receiving Mississippi River water from the Caernarvon, West Pointe a la Hache (WPH), and Violet river diversions. The Caernarvon study sites had wetland surface elevation change rates ranging from 0.16±0.31 to 0.42±0.21 cm y−1. Vertical accretion ranged from 0.75±0.04 to 1.57±0.05 cm y−1, and shallow subsidence ranged from 0.59 to 1.21 cm y−1. Wetland surface elevation at the WPH study sites initially increased 2.3 to 3.3 cm during the first seven months of the study and then steadily decreased over the following year. The overall rate of elevation change ranged from 0.27±0.09 to 0.70±0.11 cm y−1. Vertical accretion and shallow subsidence ranged from 1.24±0.08 to 1.84±0.07 cm y−1 and 0.54 to 1.27 cm y−1, respectively. The Violet sites lost elevation and had the highest subsidence rates in this study, most likely due to a combination of hydrologic alteration and low diversion discharge. Wetland elevation decreased throughout the study, with rates ranging from −1.10±0.24 to −2.34±0.41 cm y−1. Vertical accretion and shallow subsidence rates at the Violet-Near and Far sites were 0.44±0.10 and 0.44±0.11 cm y−1 and 2.78 to 1.54 cm y−1, respectively. The Violet-Mid site wetland was burned in Winter 1999, leading to more than 4.0 cm decrease in material measured over the marker horizon and contributing to the lowest accretion rate measured in this study of 0.34±0.05 cm y−1. Analysis of regional relative sea-level rise (RSLR) indicates that all Caernarvon sites and the WPH-Near and Mid sites are keeping pace with RSLR. This study indicates that the use of river diversions can be an effective coastal restoration tool, with efficiency related to the proximity to riverine source and degree of hydrologic alteration, quantity of river water released, and land uses of the receiving wetland basin. Landscape modifications such as spoil banks associated with oil and gas access canals negate the benefits of river water introduction by limiting wetland-water interaction and should be removed in conjunction with river diversion implementation for effective wetland restoration.