Record 2011 Spring Flood of the Mississippi River: How Much Nitrate Was Exported from Its Largest Distributary, the Atchafalaya River, into the Gulf of Mexico?

Abstract

The Mississippi River Flood of 2011 was a record-breaking flood that required the opening of the Morganza Spillway for only the second time since its completion in 1954. The opening brought a large quantity of nitrogen-rich Mississippi River water into the Atchafalaya River, causing widespread inundation in the river corridor wetlands and flood plains. A rapid sampling study was conducted from May 14 through July 20 at the Atchafalaya River’s inflow and outflow locations to determine nitrate transport and retention of the basin during a historic flood event. The goal was to test the hypothesis that riverine wetlands and floodplains in the Lower Mississippi River Basin (LMRB) function as a significant sink for nitrate during an extreme flood event. Based on intensive sampling data, a total nitrate mass of 97,100 Mg entering and 93,500 Mg leaving the Atchafalaya for the 10-week high flood period was estimated. This large quantity of nitrate export represents approximately 54% of the reported long-term average annual nitrate output from the Atchafalaya River Basin. The marginal 4% mass reduction during such a record flood suggests that riverine wetlands and floodplains of the LMRB require substantial engineering to become an effective sink for riverine nitrate.