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ABSTRACT: Managing the Sediment and Water Surplus of the Mississippi and Atchafalaya Rivers for Wetland Restoration

Joann Mossa

Because navigation and water supply in the Mississippi-Atchafalaya system must be maintained, use of water and sediment for wetland restoration in coastal Louisiana will be restricted to times of surplus flow. This paper characterizes sedimentary processes in the system to describe how to maximize the effectiveness of diversions when surplus flow is available. Considerations of river behavior include differences in sedimentary processes between years dominated by low discharges and years with high discharges, the most effective discharges for sediment transport, downstream variations in hydrologic and sedimentary processes, differences in relationships of discharge with the sand and silt-clay components, and seasonality. Human-induced changes that may affect these factors include engineering projects on tributaries and the main system.

The Mississippi-Atchafalaya system operates very differently, and therefore should be managed very differently, during high and low flow years. Specifically, if maximum sediment concentrations are desired, during high flow years water should be diverted during the rising limb of the hydrograph, with releases earlier on the rising limb with floods of larger magnitude. Diversions during low flow years should be coincident with discharge maxima, and are not recommended during extreme low flows. The most effective discharge levels for releasing those flows are above 400,000 cfs on the Mississippi and 200,000 cfs on the Atchafalaya, if it is desired to maximize the suspended sediment concentration. Sediment shows different responses at different locations, and management entails considerin differing local conditions. Seasonality of flow and sediment volumes is also an important factor in management, because most of the sediment is supplied to coastal areas during the winter and spring months, particularly during high flow years.

Human-induced changes to date have had a profound influence on sediment supply and the distribution of water and sediment in the system, particularly the reservoirs and flood control system. Future impacts include diversions and the construction of a series of locks and dams on the Red River. Although it is unknown how many diversions will be constructed in the Mississippi, with reduced flows sediment in suspension will tend to settle and concentrations will decrease. Modification of the Red River will have a large impact on sediment supply in the Atchafalaya, as it provides only 20 to 40% of the flow, but often comprises over 50% and sometimes over 70% of the sediment supply, to the Atchafalaya. Sediment supply to coastal areas from both rivers may therefore be reduced as a result of human activities.

AAPG Search and Discovery Article #90999©1990 GCAGS and Gulf Coast Section SEPM Meeting, Lafayette, Louisiana, October 17-19, 1990