NEW ORLEANS -- A major flood control structure on the Mississippi River may have to soon be opened up to ease pressure on New Orleans levees, federal authorities said Tuesday.

The river is expected to crest above 17 feet (5.2 meters) at a key New Orleans gauge as early as Friday, the U.S. Army Corps of Engineers said in a news release.

That would likely lead to the opening of a structure that diverts water through the Bonnet Carré Spillway. It would be the first time the structure has been used three years in a row to keep the river from damaging New Orleans' levees.

The spillway was created to limit the river's rush past New Orleans, keeping it below 1.25 million cubic feet (35,400 cubic meters) per second. But opening the spillway has environmental effects, sending river water and the pollutants it carries into brackish Lake Pontchartrain north of New Orleans and the Mississippi Sound, dropping their normal salinity levels.

Two extended openings of the structure last year were blamed by Mississippi authorities for feeding toxic algae blooms and killing oysters, dolphins and other sea life. The U.S. Commerce Department declared an economic disaster in Louisiana, Mississippi and Alabama because of “extreme flooding events in the Gulf of Mexico.”

A federal lawsuit filed in Mississippi in December by several cities, counties and groups accuses the corps and commission of violating federal law by opening the spillway more frequently. They say the defendants hurt wildlife and localities by opening it for prolonged periods of time without considering the consequences.

Joe Spraggins, who heads the Mississippi Department of Marine Resources, told The Sun Herald in Biloxi, Mississippi, that he hopes the Bonnet Carré will be opened for fewer days this year. He said damage would be less if the spillway could be closed by the end of April. He expects problems if the fresh water flow continues as the sound grows warmer.

The Corps said Tuesday's announcement was the result of an unexpectedly high forecast for the river's crest. The statement stressed that the forecast could change.