Low water on the Mississippi River and a less active steel industry caused the third straight monthly drop in cargo movement at the Port of Vicksburg in September, officials said.

Steel coils and corn made up the 12,467 tons unloaded at the port’s support platform for the month, down nearly a third from August, according to Kinder Morgan. Totals have not rebounded to reach June’s 32,046 tons or the year’s highest monthly haul, 51,799 reported in March.

“It’s mainly due to river levels,” port director Wayne Mansfield said.

The drought-shrunken river was 2.2 feetin Vicksburg this morning, unchanged from Monday, and about the same as levels measured at the end of September. Predictions into late October and early November have gone up and down, notably in the weeks after Tropical Storm Isaac. The river is projected to dip to minus 0.2 feet by Nov. 9, according to the National Weather Service Lower Mississippi River Forecast Center.

Demand is short in the steel industry and products are moving slower at the moment, said Tom Murphree, a regional sales manager for Kinder Morgan, the port’s contract operator.

“It will probably be a couple more months before Severstal picks up again,” Murphree said, referring to the Russia-based steel maker that uses the river as an avenue to move material for its Lowndes County plant. “It’ll be slower than what we’ve seen earlier in the year.”

Cargo shipments so far this year are on pace to double 2011’s total. Through September, 272,815 tons have been unloaded, versus 146,137 tons last year when the port closed during the river’s historic flood.