Deltaic development in the mouths of the Mississippi and Atchafalaya Rivers (MAR) has been found to be strongly affected by river floods. Although several studies have investigated long-term total suspended solids (TSS) discharged from the Mississippi River main channel and its distributary channel, the Atchafalaya River, little is known about sediment fractionation in TSS loads under various hydrologic regimes As the importance of sand transport for the MAR deltaic development is increasingly recognized, understanding of riverine sediment behavior is crucial for future river engineering and sediment management. This study utilized long-term (1973 – 2012) records on discharge and sediment at Tarbert Landing of the Mississippi and at Simmesport of the Atchafalaya to assess behaviors of sands and clay/silts to the flow regimes. We found that clay/silt concentrations at both locations increased rapidly with increasing discharge up to ~120% at their peak average concentrations (~270 mg L-1 at Tarbert Landing; ~350 mg L-1 at Simmesport), but followed by an almost linear drop to the low level at low flows (~130 mg L-1 at both Tarbert Landing and Simmesport). Sand concentrations at the two locations responded to the river discharge differently: with increasing discharge, the sand concentration increased by over ~900% (from ~10 mg L-1 at both locations to ~90 mg L-1 at Tarbert Landing and to ~120 mg L-1 at Simmesport) and remained almost constant at the level throughout the high flow period. Because of these sediment-river flow behaviors an overwhelmingly large percentage of annual sand loads was delivered during a few short periods of flood events. These findings highlight the need to regulate river flow in the lowermost Mississippi-Atchafalaya River Basin to maximize sediment sources.
Key words: riverine sediment, sand load, river floods, Mississippi-Atchafalaya Rivers