THE FATE AND TRANSPORT OF SUSPENDED SEDIMENT IN THE LOWER MISSISSIPPI-ATCHAFALAYA RIVER BASIN DURING THE 2011 MISSISSIPPI RIVER FLOOD, APRIL THROUGH JUNE

Since the late 1950s, a reduction in annual suspended-sediment loads has been documented in the lower Mississippi River main stem likely a result of the construction of dams in the Missouri River Subbasin and the implementation of agricultural management practices on lands within the Mississippi River Basin. Consequently, the reduction in sediment has led to a loss of coastal wetlands in Louisiana and land subsidence which some believe has made the area more vulnerable to effects from hurricanes. In April through July 2011, the lower Mississippi-Atchafalaya River Basin (MARB) experienced a flood of historic proportions. Since the flood occurred at a time of year when agricultural fields were being tilled, agricultural lands were thought to be more susceptible to overland runoff of sediment and agricultural chemicals. In order to characterize the suspended sediment of the river during the flood, the U.S. Geological Survey collected water samples at eight sites on the main stem of the lower Mississippi River, three sites on the Atchafalaya River, and at the three major flood-control structures. Additional water samples were collected at these sites for the analysis of field properties, nutrients, and pesticides. Suspended-sediment concentrations measured during the flood were similar to concentrations that had been measured historically in the lower MARB; however, fluxes were quite high because of the high streamflow. The majority of the suspended-sediment flux introduced into the basin during the flood was from the upper Mississippi River Basin despite that the majority of the flow introduced into the lower MARB was from the Ohio River, and that the previous construction of reservoirs on the Missouri and Arkansas Rivers had cut off the largest natural source of sediment to the Mississippi River. Also, approximately two times more suspended sediment entered the Gulf of Mexico from the Atchafalaya River Basin than from the main stem of the Mississippi River as observed in previous studies. High streamflow associated with the flood resuspended coarse-grained bed material resulting in 2.2 million metric tons more suspended sediment leaving the lower MARB than had entered from the Upper Mississippi and Ohio Rivers combined.