During a normal year, National Geographic reports, the saltwater wedge only makes it to 50 miles upstream. As of August 24, the Army Corps reports the leading edge of the Gulf waters to be 89.8 miles upstream. At 95 miles upstream, there is an intake pipe that supplies fresh water to New Orleans. This is a problem.

The intake pipe is not on the bottom of the river; the saltwater wedge would have to progress 20 or so miles beyond it before salinity becomes an issue. But saltwater could flow upwards at a rate of 2-3 miles per day under the right conditions. To ensure that doesn't happen, the Army Corps is building an $8.1 million underwater sill (shown in the diagram below) to stop the flow of the salt water, at a location 64 miles north of the mouth of the Mississippi.

Although the diagram doesn't quite show it, the corps has reported it might need up to 2.7 million cubic yards of dredged sediment to build the sill. You could fill more than half of the New Orleans Superdome with that amount of sludge. Once in place, it would halt the upward flow of the Gulf in two weeks. When the river output increases again, the sill will naturally degrade.

But there is another factor to contend with: Hurricane Isaac. The oncoming storm could affect the operation in two ways. One would mitigate the problem; the other would make it worse.

"There is no one size fits all answer to what effects a hurricane could have on the saltwater issue," Rachel Rodi, an Army Corps spokesperson told me via email. "Every storm is different and there are too many variables to pin down. For example: while the storm surge may temporarily raise salinity levels in the area, the rain associated with a land falling storm would make its way up the Mississippi River Valley, end up draining back into the river, and raise flows upstream that would help push the saltwater wedge back down river."

The Mississippi is more human-engineered than a natural river at this point. As Alexis Madrigal put it: "The Mississippi no longer fits the definition a river as 'a natural watercourse flowing towards an ocean, a lake, a sea, or another river.' Rather, the waterway has been shaped in many ways, big and small, to suit human needs."

But weather still matters, and effects the daily operations of the river. While it is a heavily engineered and meticulously maintained piece of transportation technology, at the end of the day, the mighty Mississippi is still beholden to floods, droughts, and hurricanes.

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