“Construction could begin as early as Wednesday on a $5.8 million underwater barrier to block the upriver flow of saltwater in the Mississippi River that is threatening area water supplies. The 1,700-foot-long underwater dam will be built at Alliance in Plaquemines Parish, using sediment dredged from the river bed just upstream.
“The sill will fill the deepest portion of the river, including the navigation channel, to bring it to a depth of no higher than 45 feet below sea level, which will allow shipping to continue to use the channel.
“The dredging operation will be coordinated with shipping interests to avoid obstructing the movement of ocean-going vessels on the river, corps officials said. Similar sills were built in 1988 and 1999.
“Saltwater from the Gulf of Mexico is denser than the freshwater carried by the river and is flowing north beneath the freshwater because the river’s flow is unusually low, measuring 2.4 feet above sea level at the Carrollton Gage in New Orleans on Monday. It is forecast to continue dropping and stay as low as 1.5 feet at Carrollton through mid-September, according to the National Weather Service’s Lower Mississippi River Forecast Center.
“The bottom of the river’s channel just south of Natchez, Miss., actually is lower than the surface of the Gulf of Mexico below New Orleans, which allows the saltwater to flow upstream. Last Wednesday, the corps measured the leading edge, or toe, of the sill at mile marker 86.5, or just south of Meraux.
“The low river conditions are a byproduct of drought conditions that have hit the Midwest this year, resulting in little rainfall in the river’s upper watershed. While the New Orleans area’s rainfall of 41.57 inches through yesterday is near the average for 1981 to 2010, very little rainfall runoff enters the Mississippi below Baton Rouge.”
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