Floodplain biogeochemical processing of floodwaters in the Atchafalaya River Basin during the Mississippi River flood of 2011

The 2011 flood in the Lower Mississippi resulted in the second highest recorded river flow diverted into the Atchafalaya River Basin (ARB). The higher water levels during the flood peak resulted in high hydrologic connectivity between the Atchafalaya River and floodplain, with up to 50% of the Atchafalaya River water moving off channel. Water quality samples were collected throughout the ARB over the course of the flood event. Significant nitrate (NO₃-) reduction (75%) occurred within the floodplain, resulting in a total NO₃- reduction of 16.6% over the flood. The floodplain was a small but measurable source of dissolved reactive phosphorus (SRP) and ammonium (NH₄+). Collectively, these results from this large flood event suggest that enhancing river-floodplain connectivity through freshwater diversions will reduce NO₃- loads to the Gulf of Mexico during large annual floods.