The Mississippi River (MR), draining 41% of the continental United States, delivers each year approximately 953,000 megagrams (Mg) nitrate-nitrogen (referred to as nitrate or NO3N from here on) [Goolsby et al., 2001] into the Northern Gulf of Mexico (NGOM). About 174,600 Mg of the nearly 1 million Mg of nitrate input is discharged from the MR's largest distributary, the Atchafalaya River Basin (ARB) that has extensive floodplains and backwater swamps [Xu, 2006]. The excess nitrogen is one of the major causes of the hypoxic dead zone [a condition when dissolved oxygen (DO) concentration in the deepwater is below 2 mg L−1] occurring in NGOM during late spring and summer for the past two decades [Rabalais et al., 2007; Turner et al., 2008]. Ecologically and economically, the hypoxic dead zone can have large reaching effects [O'Connor and Whitall, 2007; Diaz and Rosen, 2011]. Rabalais et al.  found that the extent of hypoxia in July averaged 13,500 km2 from 1985 to 2009, with a range from negligible in 1988 to 22,000 km2 in 2002. The fluctuation of the hypoxic dead zone has been found to be partially dependent on nitrogen load from the upper MR (UMR) [Wang and Justic, 2009], especially during May and June [e.g., Rabalais et al., 1996] when river flow is normally high. To reduce the large nitrogen input to NGOM, several options were suggested in the action plan released by the Mississippi River/Gulf of Mexico Watershed Nutrient Task Force , including diversion of the nitrogen-rich Mississippi water into floodplain wetland systems.
 Many studies have found that riverine corridor wetland systems have the capability of reducing nitrogen loading to downstream areas [e.g., DeLaune et al., 2005; Noe and Hupp, 2009]. Floodplain systems have been reported to be effective sinks for riverine nutrients through removal mechanisms including denitrification, assimilation, and subsurface transport [Lindau et al., 1994; Tockner et al., 1999; Forshay and Stanley, 2005]. However, it has also been reported that denitrification in river sediments is rather low because of unfavorable conditions [e.g., Hill, 1979; Alexander et al., 2000]. Conditions that favor denitrification include high concentrations of nitrate and organic carbon with high water temperatures under anoxic conditions [Pina-Ochoa and Alvarez-Cobelas, 2006]. Of these conditions, nitrate concentration in the overlying water was determined as the dominant control on denitrification potential followed by the thickness of the oxic soil surface layer [Christensen et al., 1990]. Racchetti et al.  argued that riverine wetlands increase interaction surface for denitrification while supplying nitrate constantly and, therefore, encourage higher rates of nitrogen removal.
 Channels of most rivers today are confined by levees for flood control and navigation purposes. The confinement separates the rivers from their natural floodplains, limiting or eliminating element exchange between water and terrestrial systems. This is particularly the case with large river systems, such as the MR, whose current path is estimated to cover only 10% of its once vast floodplain. Alexander et al.  reported that nitrogen loss by denitrification decreases with increasing channel size; therefore, despite the AR's potential for denitrification, it will occur when the channel water interacts with its extensive floodplain. According to our previous sampling from the Atchafalaya [BryantMason et al., 2012], this may be limited to very high flood stages, higher than typically seen in the yearly spring floods. With little progress made in reducing nitrate transport in the MR and in some locations nitrate increasing [Sprague et al., 2011], determination of nitrate reduction techniques, especially during high flow events, is vital.
 Although the AR would appear to be an ideal area to reduce nitrate loading from the MR, it does not do so under average conditions when examining the annual NO3N budget [Xu, 2006; Turner et al., 2007]. A significant flooding event should in theory allow the river to leave the channel to interact with high denitrification-potential hotspots found in the basin by Scaroni et al. . The 2011 major MR flood provided a unique opportunity for us to conduct a rapid sampling to test the hypothesis that floodplains function as a significant sink for nitrate during an extreme flood event. Combined with mass balance data, paired isotope technique can determine removal processes such as assimilation and denitrification [e.g., Wassenaar, 1995; Cohen et al., 2012]. We also aimed to assess what role the timing of the flood later in the season played in nitrate removal. During normal river flow conditions, there is low denitrification potential resulting in nitrate loads, , and values being equal at the input and output [BryantMason et al., 2012]. We hypothesized that during extreme flood events, overbank flow occurs and the river water interacts with the floodplain where there is higher denitrification potential. As a result, the nitrate loads will be lower at the output and the and will be higher at the output reflecting denitrification.