Recent decadal growth of the Atchafalaya River Delta complex: Effects of variable riverine sediment input and vegetation succession

Abstract

The Mississippi River Delta Plain has experienced substantial wetland loss from subsidence, erosion, and sea level rise, threatening coastal communities and the ecosystems that support them. The Atchafalaya River, the largest distributary of the Mississippi River, has one of the few prograding delta features along the ~200-km deltaic coastline. Understanding changes in the Atchafalaya River Delta complex (ARDC) development has critical implications for future prediction and management strategy for the Mississippi River Delta Plain. This study was organized to answer two major questions: (1) how did development of the ARDC respond to fluctuation in riverine sediment supply over the period 1989–2010, and (2) has vegetation succession helped stabilize subaerial land? The study quantified annual total suspended sediment yields to the two ARDC subdeltas—Atchafalaya River subdelta (ARSD) and Wax Lake outlet subdelta (WLSD)—classified delta land cover using satellite imagery over ~5-year intervals into three classes: barren land, vegetation, and open water and investigated the relationship of delta land change with sediment yield and vegetation succession. Over the entire 21-year study period, we found a net land gain of 59km2, with the ARSD accounting for 58% of this gain and WLSD 42%. Sediment yield to the subdeltas decreased from an average annual of 38megatonnes (MT) for ARSD and 18 MT for WLSD during 1989–1995 to an average annual of 24MT for ARSD and 17MT for WLSD during 2004–2010, corresponding to the decrease in riverine suspended sediment concentration. Concurrently, total land growth rate decreased from 2.4km2y−1 to 1.6km2y−1 for ARSD and 3.2km2y−1 to 0.6km2y−1 for WLSD. However, the ARDC had a net land loss of 2.1km2 during 1999–2004 because of tropical system effects in conjunction with the lack of large river floods (defined as discharge>13,800m3s−1). On average, more than 60% of newly vegetated land remained vegetated in subsequent years, and when compared with barren areas, vegetated land was less likely (7.3% vs. 32%) to be converted to water, indicating vegetative stabilization effect. However, during the period without a major flood, vegetation buffering against tropical system erosion was limited. This indicates that over the period 1989 to 2010 land growth of the ARDC was dictated by large flood events.