Historical sediment discharge trends for the lower Mississippi River

Abstract

The data compiled from several sources indicate that the suspended sediment load transported by the Mississippi River to the Gulf has decreased since 1950 by approximately 60%. This decrease appears to coincide with dam closures on the Missouri and Arkansas Rivers. The present volume of sediment that is available for unconfmed overbank flow into areas adjacent to the River, if not restricted by artificial levees, represents approximately 3% of the total annual suspended load. Since 1950, this would have amounted to 163 x 106 tons of sediment. The volume of sediment available at flood stage by confined flow through crevasses may almost equal the quantity available by unconfmed overbank. During the 1973 flood, the sediment volume passing through the Bonnet Carre spillway was 90% of that available by unconfmed overbank flow. There has been a decided shift in the volume of bed sediment storage from point bars to the channel floor, and this shift may allow this material to be more readily transported. Although it is difficult to quantify, one suspects that the major decrease in suspended sediment load would also be reflected by a similar decrease in bed material. Whatever changes have occurred in the upper section of the River, evidence indicates that bed sediments have been accumulating in the lower section of the River between Old River and New Orleans. This has resulted in aggradation within the thalweg of over 1.8 mlkm of channel since 1880. Only minor aggradation is evident below New Orleans. Thus, the sediment accumulation above New Orleans appears to represent a wedge of sediment that is or will migrate downstream to the delta possibly without any tendency for sediment storage in this lower segment. Grain size analyses indicate that there has been a fining of sediments within both the suspended and bedload fractions of the lower River. The loss of this coarse material may play an important role in maintaining subaerial land surrounding the delta front