Decreasing land growth and unique seasonal area fluctuations of two newborn Mississippi subdeltas

A total of 491 Landsat satellite images from 1973 to 2017 were used to study the evolution of two newborn subdeltas of the Mississippi-Atchafalaya system, the Wax Lake Delta (WLD) and the Atchafalaya Delta (AD). The exposed delta areas derived from the satellite images using an unsupervised classification method were applied to evaluate the long-term land growth and seasonal area variations in the WLD and AD, as well as their main influencing factors. The results show that the land growth rate of the WLD decreased relative to the mean sea level from 1.67 km(2)/year to 0.35 km(2)/year from 1983-1999 to 1999-2017, and the growth rate of the AD decreased from 1.51 km(2)/year to 0.80 km(2)/year from 1973-1999 to 1999-2017. The land building of the two subdeltas during the 1980s to 1999 was more efficient than that from 1999 to 2017. The significant decreases in the land growth rate and land building efficiency are attributed both to the increased impact from relative sea level rise and to the decrease in river sediment discharge, and the extension of the distributary channels, as well. The satellite-derived exposed areas of the WLD and AD show unique seasonal loops affected by the growth of vegetation during the warmer weather from April to June and the decay of vegetation mainly due to the cold fronts in December, which is different from the monotonic changes in the areas of other deltas, mainly driven by the rise and fall of water levels. A new method based on the correction between the exposed areas and water levels from December to April was used to estimate the vertical growth rates of the WLD and AD. After deducting the relative sea level rise rate, the net sediment deposition rates of the WLD and AD were estimated to be 20.8 and 24.2 mm/year, respectively. Approximately 72% and 62% of the sediments deposited in the WLD and AD were used to offset the effects of relative sea level rise when building deltaic lands from 1999 to 2017. This study not only improves our understanding of the growth of newborn subdeltas but also aids in assessing whether river course shifts and sediment diversions can restore deltas or deltaic wetlands that are at risk of drowning. (C) 2021 Elsevier B.V. All rights reserved.