And that's the last thing the city needs as it braces for Hurricane Isaac.

Saltwater has been flowing from the Gulf up the Mississippi River, and that's as much of a problem as you might think.

Due to the historic drought still searing the Midwest and taxing the heart of our agricultural economy, the Mississippi River is at its lowest flow rate in years. In Memphis, for instance, the river is 8.9 feet below baseline. While the low flow has been grounding barges and creating pockets of quicksand, it is also allowing the Gulf of Mexico to flow nearly 90 miles upstream from the mouth of the Mississippi.

Why? Salt water is denser than fresh water, which means if you mix the two, the salt water sinks. The Gulf is salt water. The bottom of the Mississippi is lower than the surface of the Gulf. Following that gradient, the dense gulf waters actually flow up the Mississippi, crawling along the riverbed. Theoretically, the salt water can flow 350 miles from the mouth of the Mississippi, the point where the bottom of the river reaches an elevation higher than the surface of the Gulf. But the salty waters never makes it this far, as the downward flow of the Mississippi holds them back.

A diagram from an Army Corps report demonstrates the salt-fresh water interface crawls along the riverbed.


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.