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The Mississippi River is a connected system. If farms dump fertilizer into the river’s tributaries in Ohio, that waste ends up in the Gulf of Mexico. If heavy rains make the river’s level rise in Illinois, it puts pressure on flood levees in Arkansas. 

And if a hot, dry summer sweeps across the Midwest, drying the river out for hundreds of miles downstream, something very strange happens in southern Louisiana. 

For the past month, as the water level on the lower Mississippi River sinks lower amid an extreme drought stretching from Nebraska to Ohio, a mass of salt water has been pushing upriver from the Gulf of Mexico toward New Orleans, filling the space where fresh water should be. Salt water is heavier than freshwater, so the water forms the shape of a wedge pressing against the bottom of the river. The wedge has already slithered more than 50 miles upstream, passing several small communities in rural Plaquemines Parish, and experts say it will likely reach New Orleans by the end of the month. Even as local and federal officials rush to slow down ... Read more

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