Chicago ? The city was in a predicament. By the late 1800s, the slow-moving Chicago River had become a cesspool of sewage and factory pollution oozing into Lake Michigan, the source of drinking water for the bustling metropolis.

The waterway had grown so putrid that it raised fears of a disease outbreak and concerns about hurting development. So in a first-of-its-kind feat, engineers reversed the river by digging a series of canals that not only carried the stinking mess away from the lake, but also created the only shipping route between the Great Lakes and the Mississippi River.

Now a modern threat — a voracious fish that biologists are desperate to keep out of Lake Michigan — has spurred serious talk of undertaking another engineering feat almost as bold as the original: reversing the river again to restore its flow into the lake.

The Army Corps of Engineers is studying ways to stop invasive species from moving between two of the nation’s largest watersheds, including a proposal to block the canals and undo the engineering marvel that helped define Chicago.

After the first reversal, the city at the edge of the prairie blossomed and today is known for stunning skyscrapers, a sparkling lakefront and a river dyed green every St. Patrick’s Day in the heart of Chicago’s downtown Loop.

The idea to reverse the river again got little traction when environmentalists suggested it a few years ago. But that was before Asian carp swam to within 25 miles of Lake Michigan, where they are being held at bay with electric barriers that deliver a nonlethal jolt. And it was before a study that showed dozens of other species were poised to move between the basins.

Adding to the urgency is the discovery last month of more carp DNA, though no actual carp, in waterways just six miles from Chicago, which could indicate that some slipped through the barriers. One live carp was found past the barrier last summer, but officials weren’t sure how it got there.

The fish are rapacious eaters that can grow to 4 feet and 100 pounds, and they have been migrating up the Mississippi and its tributaries for decades. Scientists say they could decimate the Great Lakes’ $7 billion-a-year fishing industry and unravel the food web by starving out native species.

But carp are not the only threat. A corps report issued this summer identified eight other species that could enter the lakes.

What’s more, the agency concluded, the lake isn’t the only body of water in danger. The risk to the Mississippi basin is even greater because the canals offer a potential highway for about 30 species to invade the river and its tributaries from the Great Lakes.

“That was one of those ‘Aha!’ moments,” said David Wethington, who’s managing the corps study. “You hear a lot about Asian carp and the potential devastation (to the Great Lakes), but what if things go the other way?”

The idea of separating the two watersheds, which have no natural links, has gained support from powerful lawmakers, surrounding states and scientists who believe it’s the only way to avoid irreversible ecological and economic harm.

“If we don’t, a century from now, our children and grandchildren will have lakes full of invasive species … and we will be sacrificing two of the greatest freshwater ecosystems of the United States to invasion and lost economic opportunity,” said Joel Brammeier, president of the environmental advocacy group Alliance for the Great Lakes.

But the corps isn’t ready to say whether reversing the Chicago River again is the solution. Its recommendation may have to wait another four years.

To reverse the river, engineers would barricade the canals that have been used for more than a century to send the river flowing to the west. With those channels closed, the river would resume its previous course toward Lake Michigan because the river would again be higher than the lake.

And that might be the easiest part.

Industries that use the waterways to move everything from grain and road salt to coal and chemicals oppose the idea. They complain they stand to lose billions a year if they have to rely on more expensive trains and trucks.

“I don’t want the Asian carp in the Great Lakes any more than anyone else does, but (separating the watersheds) destroys the economics of moving by barge,” said Mark Biel, executive director of the Chemical Industry Council of Illinois and chairman of Unlock Our Jobs, an industry coalition that opposes the idea.

Perhaps a bigger obstacle is Chicago’s sewer system, which collects rainfall in a big part of the metro area and then discharges it toward the Mississippi. Despite billions spent on an extensive underground tunnel network, the system still cannot contain enough storm water and sewage during heavy rainstorms, forcing authorities to open shipping locks and dump the runoff into Lake Michigan to spare basements.

Reversing the river would push even more water toward downtown and the lake, possibly requiring the city to spend billions more than planned on reservoirs and pipes to hold back the flow and prevent massive flooding.