The Mississippi flowed backwards for nearly 24 hours when Hurricane Isaac roared off the Gulf of Mexico and made landfall in Louisiana, the US Geological Survey said yesterday.

In a statement, it said the Mississippi flowed upstream at 5,150 cubic metres per second on Tuesday at Belle Chasse, downriver from New Orleans, where it surged three metres above its previous height.

Average flow per second at that point on the 3,734-kilometre river, the mainstay of North America’s largest river system, is 125,000 cubic feet.

“Although it doesn’t happen often, hurricanes can cause coastal rivers to reverse flow,” said the Geological Survey, a federal agency that constantly monitors the Mississippi’s flow rate through stream gauges.

“Between the extremely strong winds and the massive waves of water pushed by those winds, rivers at regular or low flow are forced backwards until either the normal river-flow or the elev-ation of the land stop the inflow.”

As Isaac creeped further inland, the agency said, it triggered surges along the Mississippi as far north as Baton Rouge, Louisiana, where the river crested nearly 2.5 metres higher.

The Mississippi similarly went into reverse gear when Hurricane Katrina struck Louisiana with ruthless force in August 2005, cresting at nearly four metres about its previous level.