Three-decadal erosion and deposition of channel bed in the Lower Atchafalaya River, the largest distributary of the Mississippi River

Abstract

Many world's large rivers have experienced significant sediment reduction in the past century. However, channel aggradation near the mouth of these rivers may be still occurring or even accelerating, as evidenced by a recent study on the Lowermost Mississippi River. This study aims to investigate whether such a trend exists near the mouth of another large lowland alluvial river, the Atchafalaya River, which enters the Gulf of Mexico via two outlets — the natural mainstem and a man-made straight channel that is shorter and has a much greater slope. The overarching hypothesis is that, despite the reduction in riverine sediment delivery, lowland alluvial rivers such as the Lower Atchafalaya River in their backwater zone aggrade. We collected bathymetric survey records in 1977 and 2006 to analyze riverbed deformation. We utilized stage records at several gauging stations to determine the change in the hydraulic head over time. Geospatial techniques were used to develop channel bed Digital Elevation Models which were used to identify deformation of the 174-km channel bed of the Lower Atchafalaya River. We found a cumulative erosion of 6.34×107m3 and a cumulative deposition of 8.79×107m3, resulting in a net deposition of 2.45×107m3 during the study period. While much of the river including the man-made channel was in a degrading state, the last 35km of the river's natural mainstem experienced substantial aggradation. On average, riverbed elevation of the Lower Atchafalaya decreased by 1.47m in the upper 98km, but increased by 0.96m in the final 35km of the river's mainstem to the Gulf of Mexico. The channel erosion in the upstream reach will likely continue with the river discharge projected to increase, and the channel deposition in the downstream mainstem may accelerate because of sea-level rise.