ABSTRACT: Atchafalaya-Wax Lake Deltas: An Update on Geologic and Oceanographic Impacts of the Latest Mississippi River Delta-Switching Event
ROBERTS, HARRY H., NAN WALKER, ROB CUNNINGHAM, and SUSAN MAJERSKY
This youngest episode of "delta switching" started centuries ago but because of inland sedimentation it has resulted in delta-building at the coast only since the 1950s. Data from high altitude color-infrared photography acquired by NASA and USGS plus hydrographic data from Atchafalaya Bay indicate that as of 1994 the new deltas building at the lower Atchafalaya River and Wax Lake Outlets comprise 125.8 km2 of new land above the -0.3 m (-1 ft) datum (approx. mean low tide). Since 1981 growth of the Wax Lake delta has averaged ^sim 2.9 km2/yr above this reference level. The lower Atchafalaya River delta, which was the first to appear (in 1973), has grown at a slower average rate of 1.9 km2/yr over this same period. The Wax Lake delta did not start rapid growth until about 1981 because of sedimentation in local inland basins (e.g., Wax Lake). Growth of the Atchafalaya delta has slowed because of the efficiency of sediment transport to the adjacent continental shelf via a dredged navigational channel coupled with siltation of secondary channels. Both deltas are comprised of sand-rich lobes (3-4 m thick) that systematically fuse to form rapidly expanding deltas that have nearly filled Atchafalaya Bay. Suspended sediments that by-pass the deltas form a distinct plume that extends far beyond Atchafalaya Bay. During medium-to-high discharge conditions (> 4250 m3/sec) the plume may extend up to 50 km and cover an area of over 7400 km2. Plume area and location is highly variable and depends largely on wind direction, speed, and duration. Suspended sediment deposition is impacting the shelf and the downdrift Chenier Plain coast.
AAPG Search and Discovery Article #90941©1997 GCAGS 47th Annual Meeting, New Orleans, Louisiana