DISTRIBUTARY MOUTH BAR FORMATION AND CHANNEL BIFURCATION IN THE WAX LAKE DELTA, ATCHAFALAYA BAY, LOUISIANA

The Mississippi River has undergone at least seven cyclic avulsions during the Holocene epoch. The latest avulsion has taken a path down the Atchafalaya River into the Gulf of Mexico. Presently, two bayhead deltas are prograding into Atchafalaya Bay through a series of distributary channel bifurcations. Channel bifurcation and sediment transport processes and responses were investigated from proximal to distal ends in the Wax Lake Delta using channel flow velocities, suspended sediment data, cross-channel bottom profiles, and short push-core stratigraphy during flood and non-flood conditions. Center channel flow velocities averaged two times higher during flood conditions than during non-flood conditions. All velocities maintained near constant values from proximal to distal, then dropped sharply at each distributary channel mouth. Cross-channel flow velocities reached a maximum at or near the thalwag. During non-flood conditions, flow velocities were inversely proportional to tidal fluctuations and were greatly reduced in the face of strong southerly winds; however, tidal and wind influences were negated by flood condition flow velocities. Homogeneous suspended sediment concentrations with mean grain sizes of coarse silt to very fine sand were found throughout the system, indicating well mixed, turbulent flow. Suspended sediment concentrations were approximately 20 times higher during flood than during non-flood flow conditions. Bedload sediment size remained near constant throughout the system in all samples from proximal to distal end. These data indicate sediment moves efficiently through the deltaic system with very little grain size fractionation in suspended or bedload sediments from proximal to distal end. Downstream sediment fluxes vary directly with velocity. Thus, the thalwag transports the highest volume of sediment per unit time even though the sediment concentrations per unit volume are homogeneous. Sediment deposition per unit time is greatest at the distributary mouth channel thalweg, where velocities slow, creating a distributary mouth bar and subsequent channel bifurcation.