The impacts of the great Mississippi/Atchafalaya River flood on the oceanography of the Atchafalaya Shelf

Abstract

Rivers are the primary means by which water, sediment, and dissolved material are transported from the continents to the ocean. Despite previous advances, much remains to be learned about the dynamics of large shelf-discharging rivers, and their functional differences with deep water-discharging rivers, particularly with respect to the distribution of sediments in the coastal zone. The great Mississippi/Atchafalaya River flood of 2011 provided an excellent opportunity to examine the impacts of a large, shelf-discharging river on the coastal ocean, and the role that event pulses from such rivers play in the delivery of sediment to the inner continental shelf. Vessel-based surveys were conducted on the inner-continental shelf within the Atchafalaya and Mississippi River plume regions, providing in situ measurements of salinity, temperature, dissolved oxygen, turbidity, particle size, and current velocity profiles. MODIS satellite images and 7Be measurements were used to assist in data interpretation.We demonstrate that the Atchafalaya River plume produced intense vertical gradients in temperature, salinity, oxygen and turbidity. This occurred despite the shallow bathymetry of this system and the presence of winds, which alternated between onshore to offshore, and that might have otherwise mixed systems with less freshwater. Sedimentation rates along the inner-continental shelf were about 5–10 times greater than those measured previously during smaller “typical” floods. This large deposit is likely to be preserved, at least in the near term, because sedimentation occurred beyond normal depths of wave reworking and the intense stratification induced by this flood likely reduced mixing at the time of sedimentation. A sediment budget for this system reveals that sediment fluxes to the coastal zone during 2011 were similar to those observed in previous years, suggesting that this system is supply limited, rather than transport limited. As such, we postulate that the major impact of this flood was to change the location of the depocenter of Atchafalaya River sediments, rather than increase the annual flux of sediments to the coastal zone. These findings imply that extreme flood events may not be an ideal analog for coastal restoration along this deltaic coast.