ABSTRACT

The abnormally high flood of 1973 initiated the subaerial phase of delta development in Atchafalaya Bay by scouring sands stored in the lower Atchafalaya River channel and redepositing this sediment in the bay. Peak flows to Atchafalaya Bay in 1973 exceeded 900,000 cfs (25,496 cms), about twice the average peak flood levels at these sites. Since 1973, deltas have dramatically expanded from both lower Atchafalaya River and Wax Lake Outlets and now comprise approximately 153 km² (59 mi²) of new land above the -0.6 m datum. Data from 1994 indicate that the Atchafalaya delta accounts for 101.5 km² (39 mi²) while the Wax Lake delta has an area of 51.1 km² (19.7 mi²). These two deltas have prograded as silt/sand-rich wedges composed of numerous bifurcating channels separating sandy lobes. Sedimentologically, both deltas display the typical coarsening upward trend, but with a very thin (< 1 m) prodelta facies above the highly burrowed, and shell-rich bay bottom sediments. However, the deltas do have differences. The Wax Lake delta is still in the channel elongation-bifurcation and bar formation phase while the Atchafalaya delta is experiencing channel abandonment, lobe fusion, and bypassing of sediment to the continental shelf (via the dredged navigation channel). Rate of Wax Lake delta growth has increased since about 1980 so that subaerial exposure now rivals the Atchafalaya delta. However, a weir was installed in 1988 to direct more discharge down the Lower Atchafalaya River Outlet causing a reduction in the growth rate of the Wax Lake delta and coincident increase in Atchafalaya delta growth. In 1994, the weir was removed and the response of the two deltas to this change is being monitored. Zones of scour between the two emerging deltas decreased between 1981-1994, but scour has increased elsewhere on the perimeters. Fine grained sediments by-passing the bay have produced significant deposition on the shelf and along downdrift coasts over the past two decades, but only within the last five years has a significant amount of sand begun to reach the shelf.