Long-term Sediment Accumulation in Mid-channel Bars of the Upper Reach of the Lower Mississippi River.

A recent study reported that about 44% of the total Mississippi River suspended load reaching the Old River Control Structure (ORCS) was trapped upstream of the Gulf of Mexico by overbank storage and channel bed aggradation. Considering an average annual sediment load of 120 million metric tons passing ORCS to the Mississippi River main channel, the trapped sediment load would be equivalent to annually rebuilding 44-km² coastal land of 1 meter in depth, assuming a sedimentation bulk density of 1.2 tons m^-3. No study has yet demonstrated such a high sediment accumulation rate within the confined river channel or on a floodplain area that surrounds the only unleeved stretch (~30-km long) of the Lower Mississippi River downstream of ORCS. In this study, we utilized satellite images taken from 1983 to 2013 and analyzed changes in surface area of nine major mid-channel and point bars over a ~130-km river reach from ORCS to Baton Rouge. Using river stage records and the estimated surface areas, we developed a stage – surface area rating curve for each of the bars and estimated changes in bar volume over time. We found that more than half of the bars have grown, while the others have shrunken in the past three decades. As a whole, there was a substantial net gain of surface area and volume accretion. Sediment trapping was most prevalent during the spring floods, especially during the period from 2007 to 2011 when two large floods occurred. This paper presents the channel morphological change and sediment accumulation rates under different flow conditions, and discusses their implications for the current understanding and practices of the Mississippi River sediment diversion.