The major 2011 Mississippi River (MR) Spring Flood inundated much of the river's largest distributary basin, the Atchafalaya River Basin (ARB) with a large quantity of nitrogen-rich water. The event provided an opportunity to test the hypothesis that river corridor wetlands and floodplains function as an effective sink for riverine nitrate during an extreme flood event. Water samples were collected from 15 May to 20 July from three sites on the Atchafalaya to determine how nitrate concentration and nitrate isotopes changed from upriver to 182 km downriver. During this period, a total nitrate-nitrogen (NO3N) mass load of 89,600 megagrams (Mg) entered the basin and 83,200 Mg exited the basin, resulting in 7% retention of NO3N. Although this rate is more than previously reported for this basin, the nitrogen removal rate is lower than expected. During the high flood period, we found little change in isotope and values between the upriver site and the river's two outlets, further indicating limited nitrate processing in this large river basin with extensive floodplains and backwaters. The result strongly suggests that flow-through systems such as river corridor wetlands and floodplains may be ineffective in reducing riverine nitrate during a flood event because of limited residence time. However, there was a clear difference in isotope values between the river rising and recession periods. increased from 5.8‰ as the river rose to 7.5‰ in the flood recession, suggesting higher nitrate leaching from subsurface soils during the post-flood period from the upper MR.