“The President of the Mississippi River Commission Maj. Gen. Michael J. Walsh has directed the New Orleans District Commander Col. Ed Fleming to be prepared to operate the Morganza Floodway within 24 hours. The operation will include the deliberate and slow opening of the structure.” —U.S. Army Corps of Engineers announcement, May 13

On Saturday at 3:00 Central Standard Time the U.S. Army Corps of Engineers opened the first 25-foot-wide bay of the Morganza Spillway for only the second time in the structure’s nearly 60-year history. The last time was in 1973. The primary objective of the opening is to reduce the likelihood of the swollen Mississippi River flooding the port cities of Baton Rouge and New Orleans downstream.

If the Corps had not opened the Morganza, reports Mark Schleifstein of the Times-Picayune, the river would have crested at 19.5 feet at New Orleans, “only a half-foot below the tops of levees and floodwalls in the city.”

The Corps announced that, at least for now, the Morganza opening will be smaller than was suggested earlier: present plans call for 125,000 cubic feet of water per second to be diverted into the Atchafalaya River and Basin instead of the earlier projection of 300,000 cfs. That is less than one-quarter of the total capacity (600,000 cfs) that the spillway could divert if all 125 bays were opened. The opening of the bays will be gradual, beginning with one or two at a time in order to let the Atchafalaya fill as gently and slowly as possible; there will not be a torrent or tsunami rushing into the Atchafalaya Basin.

Flooding will still be considerable, however, over some 3,000 square miles of Acadiana. In some areas the water will be 25 feet deep. About 25,000 people will be affected. Governor Jindal has directed parish administrators in the Atchafalaya Basin parishes to direct people living in the flood plain to evacuate. “Now is the time to take action. Don’t delay. Don’t hope something will change.”

The higher water is expected to reach Morgan City within three days of the opening.

It is predicted that the Mississippi will crest at New Orleans on Monday, March 23.

With the Morganza opening, and with all of Bonnet Carré’s bays soon to be open, diverting 250,000 cubic feet of water per second into Lake Pontchartrain, the river level at the Carrollton Gauge is expected to stay at the safe 17-foot measure, with the river carrying about 1.25 to 1.5 million cubic feet per second. Anytime the river is above 17 feet at Carrollton Gauge the Corps and City and Port officials feel even more nervous about the city’s security than usual.

Further information and emergency updates are available at the City of New Orleans Emergency Preparedness / Flood Fight, at the Governor’s Office of Homeland Security and Emergency Preparedness, on Twitter at @GOHSEP, and on Facebook.

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The map above was produced by the Times-Picayune. Photographs below show the opening of the Morganza Spillway in 1973. Top photo by Associated Press; bottom photo by Pat Patterson of the Times-Picayune. Click here for Times-Picayune photos of preparations for the 2011 flood around the Atchafalaya Basin.