New Orleans may be the victim of a one-two punch as Hurricane Isaac threatens to strengthen over the Gulf of Mexico and the ongoing effects of this summer’s drought continue to trickle down to the Delta.

The record temperatures and lack of rain that have devastated crops in America’s heartland upstream also have weakened the once-mighty Mississippi River’s defenses against saltwater intrusion.

Freshwater flowing south from the Mississippi and salty water from the Gulf are constantly arm wrestling for territory in the Mississippi River Delta, where the river dumps into the sea (see map). But as dry weather shrinks the Mississippi, the Gulf is gaining ground, pushing more saltwater inland. At risk is New Orleans’ freshwater supply.

To fight back, the U.S. Army Corps of Engineers is in the midst of an $8.1-million project to build an underwater levee, called a sill. The Corps will pile up as much as 2.5 million cubic yards (1.9 million cubic meters) of mucky sediment in an effort to keep saltwater from advancing into pipes that pull freshwater from the Mississippi for drinking and industrial uses in and around New Orleans.

“It is going to block the heavier concentrated saltwater from traveling north,” said Rachel Rodi, an outreach manager with the federal agency.

Natural Tensions Exacerbated

The region is especially susceptible to saltwater inflows, in part because, for its final 350 miles (563 kilometers), the Mississippi River is already below sea level. In addition, saltwater is denser than freshwater so ocean water from the Gulf is continuously pushing north along the bottom of the river channel and under the river in the form of a wedge that drops off as it faces resistance from freshwater flowing south.

With Mississippi River levels at near-record lows (close to 10 feet lower than normal in some locations), officials are concerned the wedge will push far enough inland to contaminate New Orleans’ water supply.

New Orleans has a water intake pipe about 95 miles (153 kilometers) north of the point where the main stem of the Mississippi branches off before disappearing into the Gulf of Mexico.

The leading edge of the Gulf’s saltwater wedge is currently about 90 miles (145 kilometers) from the river mouth, just a few miles shy of the city’s supply.

There is some wiggle room, as the leading toe of the wedge would sit lower than the intake pipes.

Officials have predicted that if the wedge advances another 20 miles (32 kilometers), pushing the taller part to the intakes, that New Orleans is at risk of failing to meet U.S. Environmental Protection Agency standards for water quality.

During a summer without a drought, the wedge usually extends less than 50 miles (80 kilometers) upstream from the Gulf.

(Learn more about the drought: “July Hottest Month on Record in U.S.—Warming and Drought to Blame? “)

It is no surprise that saltwater fills the stretch of the river channel that is below sea level, according to Mark David, director of the Institute of Water Resources Law Policy at Tulane University in Louisiana.

“[Water from the Gulf] is essentially filling a container,” Davis said. The sill is designed to make the container smaller, he explained.

Building a sill barrier is not a new technique. The Corps put up the same defenses in the 1980s and ’90s, successfully protecting New Orleans’ water supply, according to Davis.

The advancing wedge has already caused towns south of the city to find new water sources.

Billy Nungesser, president of Plaquemines Parish, south of New Orleans on the Mississippi River Delta, declared a state of emergency August 7 due to increasing levels of salinity at several water intakes in the region.

The parish is now piping and barging in freshwater from upstream locations. Those who live there are asked to conserve their use of freshwater for as long as the saltwater intrudes on the freshwater supply.

(Read more about the drought’s effects: “Record Heat, Drought Pose Problems for U.S. Electric Power.”)

Sill Construction

The sill construction site is 64 miles (103 kilometers) above the mouth of the Mississippi River and below Plaquemines’ emergency supply and New Orleans’ primary supply. When finished, the structure may hold as much material as the Great Pyramid of Giza.

All of that sediment is being pumped from a site called a “borrow” that is “already environmentally cleared and has enough fill material to build with,” Corps spokeswoman Rodi said.

Dredging began August 16 and will take several weeks to complete.

The top of the sill will be 45 feet (14 meters) below the river’s surface, allowing ships to continue to use the channel. During construction, ship traffic is restricted to one lane.

The sediment being used to build the sill was previously earmarked for a wetlands restoration project.

Tulane’s Davis characterized the shifting of priorities—from wetland restoration to sill building—as part of an ongoing balancing act necessary to manage a river that fluctuates wildly with the weather.

“Big floods like last year and big droughts like this one remind us that you have to manage [the river] as a system,” he said. “And, quite frankly, if it is not working at the mouth of the river you are pretty sure that it is not working for the river system.”

Wash Out

When rains begin to fall and the river’s flow increases, the sill will wash away. The borrow site will refill with sediment over the course of two to five years, depending on river conditions and rates of sedimentation, according to Rodi.

Those rains could come soon if a hurricane like Isaac builds in the Gulf of Mexico and makes landfall near the Mississippi River, as Hurricane Katrina did in 2005. (Video: Katrina Day By Day.)

If that happens, the storm surge would push saltwater even further up the river, almost certainly raising salinity levels in New Orleans, according to Mike Stack, chief of emergency management for the U.S. Corps of Engineers.

But there might be a silver lining.

“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,” he said in an email to National Geographic News.

“That would help push the saltwater wedge back downriver.”