The diversion of nutrient-rich water through forested wetlands--especially within the Lower Mississippi River Basin--is one proposed approach to reduce nutrient export to coastal estuaries. The primary objective of this research is to quantify nutrient retention and carbon mineralization as river water is transported through the Atchafalaya Basin Floodway System (ABFS). By characterizing water transported via channelized sections of the basin in contrast to the un-channelized spillway including the forested swamps, the research team expects to find enhanced nutrient removal within water diverted through the spillway due to increased residence time and enhanced contact with floodplain sediments. Samples will be collected across the basin over the flood period (12 weeks). Water isotopes and conservative anions will be measured to quantify water sources and infer water residence time. Nutrients (phosphorus and nitrogen species) will be measured to quantify system processing and delineate spatial differences across the ABFS.
Although diverting water is a proposed approach to reduce nutrient export, and associated hypoxic zone extent, in the Gulf of Mexico, the effect of very large-scale diversion on the magnitude of nutrient reduction is not well understood. The current flooding in the Lower Mississippi (highest levels since 1927) has resulted in the Army Corps of Engineers opening the Morganza Spillway into the ABFS for the first time since 1973. The opportunity to explore the fate of the nutrient-rich Mississippi water through the ABFS is unprecedented and will provide valuable insight into the coupling of hydrology and biogeochemistry of the Atchafalaya River and Floodplain. This study will provide insights on nutrient reduction associated with large scale diversion of spring runoff water.
The majority of the annual water and nutrient export is transported through the river network during floods. Nutrient removal is known to occur within floodplains, but only when river water freely moves onto the adjacent floodplain. Our objective in this study was to quantify water and nutrient transport through the Atchafalaya River Basin (ARB) during the large flood of 2011 in the lower Mississippi River. The magnitude of the flood resulted in the opening of the Morganza Spillway into the ARB, which was only open one other time (1973). The major scientific findings of our project were as follows: The 2011 flood in the lower Mississippi River resulted in the third highest recorded flow through the Atchafalaya River Basin. During the flood, water routed from the Mississippi River via the Atchafalaya River spread across the lower ARB into backwater regions of the floodplain downstream of the main channel levees in the northern half of the ARB. Floodwater also entered the ARB from the Morganza Floodway (MF), which mixed with existing water and was transported towards the ARB outlets. At the flood peak, our results show that there was extensive mixing and hydrologic connectivity throughout the lower ARB. Up to 50% of river water moved off river and traveled through the floodplain during peak flow. Hydrologic connectivity and transport through the floodplain swamp moderated NO3- export from the ARB. The net NO3- reduction for the entire basin during the 2011 flood varied from < 10% to > 37% daily, resulting in an a cumulative removal of 14% over the 2011 water year. Our results also suggest that backwater swamps may release dissolved phosphorus if (1) there is connectivity to the river and (2) there is adequate time for development of reducing conditions that result in dissolved P release. Although the ARB is a net sink for sediment and therefore sediment-bound P, release of bioavailable P to the coastal estuary during large events may impact primary productivity including the potential for harmful algal blooms Further examination is required to determine the potential for both phosphorus release and retention. Enhancing river-floodplain connectivity through freshwater diversions will reduce NO3- loads to the Gulf of Mexico during large annual floods, that may become more common due under a changing climate. Management of connectivity between channelized flow and adjacent floodplains is an essential component of any diversion strategy that incorporates not only the benefit of lowering river stage, but also reducing downstream nutrient export.
