When will saltwater arrive in New Orleans? Here's what to know

As drought tightens its grip in the Central US and water levels on the Mississippi River plummet to near-record lows, a surge of saltwater from the Gulf of Mexico is pushing upstream, polluting drinking water for thousands of residents south of New Orleans.

With little prospect of rain in the future, officials are working to find solutions before the saltwater infiltrates treatment plants that serve tens of thousands more residents, including New Orleans.

Louisiana Gov. John Bel Edwards on Monday requested a federal emergency declaration for four parishes: Plaquemines, St. Bernard, Jefferson and Orleans.

"With the current projections, it should be noted that almost 20% of the state could be impacted by this event," Edwards said in the request, predicting saltwater could impact the state until January.

Here's what to know about the saltwater intrusion.

The Mississippi River's enormous flow rate is usually enough to keep water from the Gulf of Mexico out. But when the river is running low, saltwater can creep into it.

The saltwater wedge starts at the mouth of the river and moves upstream. Because saltwater is more dense than freshwater, it takes the shape of a wedge: The leading edge is at the bottom of the riverbed - the so-called toe of the wedge - and extends back toward the ocean.