Upstream floodwaters flowing toward a swollen Mississippi River will require the U.S. Army Corps of Engineers to open its rarely used Morganza Spillway on Sunday and funnel part of the river's flow into the Atchafalaya Basin for at least a couple weeks.

After saying two weeks ago it had "no intention" of diverting river water from Morganza to Morgan City, the Corps said last week that opening the 65-year-old spillway for only the third time was becoming more likely. The Corps announced Monday its Mississippi Valley division commander had approved opening the structure.

"The current flood fight is historic and unprecedented," Corps spokesman Ricky Boyett said in a statement. "All potentially impacted residents, landowners and businesses are urged to heed the direction of their local officials and take necessary precautions."

In a joint hearing Monday before the state Senate Natural Resources Committee and the Select Committee on Homeland Security, officials said significant ongoing precipitation in states north of Louisiana — particularly Arkansas, Oklahoma and southern Missouri — is creating uncertainty surrounding current projections on when and how much flooding could occur when all that water drains into the Mississippi River system.

The Arkansas River is been at or approaching record flood levels from Tulsa, Oklahoma, southward toward the Mississippi. Additional flooding has occurred farther north within the Mississippi basin.

Heath Jones, chief emergency manager for the Corps, said all retention ponds and other places water would saturate as levels rise are already full. So "every drop that comes down just raises the stage more."

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The spillway was previously opened for 56 days in 1973 and 55 days in 2011 — the only two times it has been used since its completion in 1954.

Jones said the Morganza Spillway contains 125 bays and officials expect to open 20 to 25 of them. The opening will happen incrementally, with one gate being opened each of the first three days, followed by additional openings that could happen at a faster rate.

The gates will then be closed gradually once water levels start to fall, but that won't be for at least a couple of weeks, Jones said. A Corps projection released last week showed an inundation lasting through late June.

The influx of water into the Atchafalaya Basin is expected to inundate cropland and camps, as well as displace and possibly endanger wildlife.

The opening date could fluctuate, or the opening could be canceled altogether, depending on whether there are any major changes to the rainfall or river forecasts.

Authorities are working to minimize the impact on nearby communities, including sinking a barge in Bayou Chene to prevent backwater flooding in the lower stretches of the spillway, including populated areas of Assumption, Iberville, St. Martin, St. Mary and Terrebonne parishes.

The lower Mississippi River typically rises and falls with the seasons, but the Corps said the current situation is "not like past river floods" because of its duration. The past nine months have been some of the wettest on record for the eastern United States — unprecedented within the past 125 years.

Boyett said Monday marks "the 214th day of the flood fight," which is expected to surpass the previous record of 225 days in 1973. He said if the Morganza Spillway isn't opened, the river is projected to overtop the structure, rendering it unsafe, by late next week. The plan for a gradual opening starting Sunday should avoid that outcome.

Jones said the Corps will hold public meetings Tuesday in Morganza and Butte La Rose and on Wednesday in Morgan City to explain its plans, field questions and "try to alleviate any concerns."

The Morganza works differently than the Bonnet Carre Spillway above New Orleans, which had to be opened on two separate occasions this year for the first time in history. The Bonnet Carre can more easily be thrown open. The Corps prefers to tread more lightly with Morganza, opening it up one gate at a time.

The opening will send river water through a structure in Pointe Coupee Parish, then on through the Atchafalaya Basin toward Morgan City and finally into the Gulf of Mexico, relieving some of the strain on Mississippi River levees downstream. The water will run from north of Morganza to the Gulf of Mexico between levees west of Livonia and east of Krotz Springs.

But the good news is that local weather has been relatively dry of late, allowing protective measures to be put in place ahead of the opening, officials said Monday.

The barge at Bayou Chene is intended to limit devastating backwater flooding when the spillway opens. Officials with the Louisiana Coastal Protection and Restoration Authority said Monday the barge is on its way to Bayou Chene and is expected to arrive Tuesday afternoon. It will then be pumped full of water and sunk in the days leading up to the Morganza opening.

The state recently approved $80 million in funding to place a permanent structure at Bayou Chene, but that construction will take several years. Installing the barge costs about $7 million.

Jim Waskom, director of the Governor's Office of Homeland Security and Emergency Preparedness, said that the state has already deployed nearly 12,000 feet of flood barrier along La. 70 north of Morgan City and it is coordinating the deployment 13,000 feet of such barriers in lower Terrebonne.

The state has also leased or coordinated with the U.S. Army Corps of Engineers for pump operations in Catahoula, Concordia, Ouachita, Rapides and St. Martin parishes and issued more than 300,000 sandbags to parishes across the state.

Gov. John Bel Edwards on Saturday requested federal aid to help the state respond to river flooding and prepare for the growing threat of rising water. The request for federal funds has not yet been approved, but officials said Monday they expect to hear back soon.

Follow Lea Skene on Twitter, @lea_skene.