Decreasing Suspended Sediment Concentrations and Loads in the Lower Mississippi and Atchafalaya Rivers, 1980 through 2015

Suspended-sediment concentration (SSC) and suspended-sediment load (SSL) trends were determined for the lower portion of the Mississippi and the Atchafalaya Rivers from 1980 through 2015 using suspended-sediment data from two stations in the U.S. Geological Survey's National Stream Quality Accounting Network. The Weighted Regressions on Time, Season and Discharge (WRTDS) model was used to derive estimates of SSC and SSL; to relate SSC and SSL with season and discharge; and to identify trends. Use of the WRTDS model reduces uncertainty in suspended-sediment concentrations related to variable streamflow conditions. Flow-normalized SSCs in each river were similar, and decreased by about half from 1980 through 2015 - from about 260 milligrams per liter (mg/L) to 130 mg/L. The annual SSL in both rivers combined decreased from about 200 million metric tons per year (MT/y) to about 100 MT/y over the same time period. SSC and SSL declined in both rivers from 1980 through 2015, although more slowly from 2005 through 2015 and these declines show signs of stabilizing. SSC and streamflow differed in an important way between the two rivers. SSC increased as streamflow increased for the entire range of observed streamflow in the Atchafalaya River. In contrast, in the Mississippi River, SSC followed the same increasing pattern during low and moderate streamflow, but concentrations decreased during the highest streamflow events which tended to occur between January and July. Since much of the water flowing in the Atchafalaya originates from the Mississippi River, the difference suggests a within-basin source of suspended sediment for the Atchafalaya River that is absent in the lower Mississippi River. Accurate estimates of the SSC and SSL available in each river are crucial for understanding how effective diversions of river water into adjacent estuaries will be in sustaining these wetlands. Our study demonstrates that there might be far less sediment available than previously reported. Further, the difference in SSC and streamflow in the two rivers is highly relevant to the ongoing discussion of coastal restoration strategies because the delta building that is occurring at the mouth of the Atchafalaya River is frequently used as a model of what could be expected with controlled diversions in the lower Mississippi River delta.