The Bonnet Carré Spillway is a flood-control structure that diverts Mississippi River water into Lake Pontchartrain during exceptionally high river stages. Because of elevated water levels in the Mississippi River in the spring of 1997, the Bonnet Carré Spillway was opened on March 17 and fully closed on April 18. The total volume of water discharged into Lake Pontchartrain was approximately 11.8 km3, or two times the volume of the lake, and the total mass of sediment discharged into the lake was approximately 7.1 × 108 kg (780,000 US tons). In 1996, 757 surface sediment samples were collected in Lake Pontchartrain and were analyzed by x-ray fluorescence spectroscopy for major cation constituents. These same sites were revisited following the 1997 Mississippi River discharge event. Analysis of the 1996 and 1997 lake-bed sediment samples was accomplished utilizing fundamental statistical and graphical methods. Element concentration contour maps and variograms for the major cations illustrate meaningful differences between the pre- and postspillway sediment samples that are not readily apparent in the analysis of the descriptive statistics alone. Major cations exhibited significantly greater spatial continuity in the postspillway samples relative to the preceding year. The concentrations of aluminum and silicon in the postspillway sediments are considered to reflect, respectively, relative variations in clay and silt contribution to total sediment. The higher concentrations of magnesium in samples collected prior to the river diversion represent adsorption of magnesium onto exchange sites in surface sediments due to exposure to more saline waters.

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