The worst is yet to come from the mighty Mississippi

The flood is expected to crest next Thursday at Vicksburg, where the Mississippi and Yazoo rivers meet, at a level easily higher than in the Great Flood of 1927

The bursting Mississippi was threatening last night to submerge still more farmland, homes and even towns as an enormous swell of water fed by spring rains and snow-melt forged its way to the Gulf of Mexico, unleashing some of the worst flooding since the Great Depression.

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While some towns already soaked by the river's wrath further to the north – including Memphis in Tennessee and Cairo in Illinois – were yesterday beginning the task of cleaning up as water levels begin to fall, the worst is still to come for low-lying areas of the Mississippi Delta and Louisiana.

In Louisiana, which has in recent years suffered Hurricane Katrina and the BP oil spill, engineers were getting ready to begin gradually opening sluices on the giant Morganza Spillway just north of the state capital, Baton Rouge. The 5,000ft span of gates has been opened only once before in history.

It is not a decision that will be taken lightly, however. While diverting some of the river's fury through the gates means lowering the risk that levees will be overtopped in Baton Rouge and further downriver in New Orleans, it will mean deliberately flooding vast areas of the state west of the river and close to the coast.

With the first gates likely to be opened at the weekend or early next week, entire cities may be forced to evacuate in the path of the escaping water, including Houma, where the BP clean-up and spill-response teams are housed, and Morgan City. As many as 13,000 buildings, 25,000 people and 3 million acres of land are likely to be impacted, the authorities said.

The situation across parts of the Mississippi Delta was already dire last night, with as many as 600 homes already touched or swamped by muddy water filled with debris and snakes. Residents were on high alert in the city of Vicksburg, where the Mississippi and Yazoo rivers merge.

The flood is expected to crest there next Thursday at a level easily higher than was seen in the so-called Great Flood of 1927.

At Natchez, Mississippi, a little further downstream, the high-water mark stood yesterday at 58.3ft. That was already higher than the 53.04ft mark set in 1937, another year of historic flooding along the river.

Residents have already abandoned their homes in the town of Tunica Cutoff, Mississippi, which last night was entirely cut off leading some in the area to predict it would be swept off the map entirely. "We don't know from day to day," said Lee Sherwin, 77, who fled Tunica Cutoff and is in a Red Cross shelter. "We don't know if we are going to want to go back or even if we can go back. I've never in my life run into a ituation like this, but we are living with it day by day."

Already some economists are warning that this year's spring floods are likely to cause damage worth between $2bn and $4bn. "Crop loss estimates are definitely around $800m for Mississippi alone," said John Michael Riley, an economics professor at Mississippi State University. The Great Flood of 1927 was estimated have cost the US about $230m, the equivalent of $2.8bn today.

The final toll will not be known until the crest finally reaches the Mississippi's mouth in just under two weeks. And much will depend on what happens at the Morganza Spillway and also how well levees and flood-protection walls elsewhere hold up.

There was concern about levees protecting low-lying communities on the Yazoo close to Vicksburg. Were they to fail, a wall of water could inundate large areas of the Mississippi Delta, already one of the most impoverished regions of the United States. Hailey Barbour, the Governor of Mississippi, urged anyone who thought their homemight be at risk to get out as fast as possible. "More than anything else, save your life and don't put at risk other people who might have to come in and save your lives," he said.

Governor Barbour also raised concern about the small numbers of families in the region who have neither phones nor electricity. "They are a tiny number but we must find them," he said.

Cotton-picking and other farms still provide most of the employment in the Delta. Nine of the 11 counties that touch the Mississippi River in the state have poverty rates at least double the national average of 13.5 per cent, according to the US Census Bureau.

While no formal decision had been made on opening sluice-gates at the Morganza Spillway, Governor Bobby Jindal of Louisiana said that people should assume that that would happen and anyone living in the path of the water once it was unleashed should evacuate the area as soon as possible.