By JIM SUHR

Associated Press

GRANITE CITY, Ill. (AP) -- Crews scrambled to make repairs Wednesday near the busiest lock on a vital Mississippi River commerce corridor near St. Louis as hundreds of barges and tugboats remained snarled in a backlog that was growing worse by the hour.

Workers closed Lock 27 last Saturday after discovering that a protection cell -- a rock-filled steel cylinder against which barges rub to help align them for proper entry into the lock -- had split open, spilling enough of the rock into the river to obstruct passage.

That damage was on an unarmored section of the vertical protection cell that the barges don't typically make contact with because they're often 15 to 20 feet under water. However, that portion has been exposed because the river's level has been lowered dramatically by the nation's drought, said Mike Petersen, an Army Corps of Engineers spokesman.

The lingering drought also has made the Mississippi narrower, leaving towboat pilots struggling to find a safe place to park their barges river as they wait out the repairs Petersen said could be completed as early as Thursday.

As of Wednesday morning, nearly five dozen tugboats and more than 400 barges -- carrying enough cargo to fill 2,400 railcars or 23,600 large tractor-trailers -- were caught up in the logjam, a Coast Guard spokesman in St. Louis said.

The number of vessels forced to park there spiked by one-third over the past 24 hours while there was a doubling of the number of barges, hauling everything from grains to coal, fertilizer and construction materials, Lt. Colin Fogarty said.

Other barges were parking at ports up and down the river, saving fuel instead of being caught up in the snarl near Lock 27.

"Imagine taking an eight-lane highway and reducing it to a four-lane one and having a massive traffic backup. That's what we're dealing with," Fogarty said. "It's like trying to find a parking spot at the mall on Christmas Eve -- chances are you're going to have to park far away, and you may not want to go at all."

Roughly half of the nation's farm exports pass through that lock now is closed at a time growers throughout the Midwest are harvesting their corn and soybeans, Petersen said.

The trouble at Lock 27 is the latest barge-related headache brought on by the nation's worst drought in decades.

Just a year since the Mississippi rose in some places to record levels, traffic along the river sometimes resembling a slow-motion freeway at times has been brought to a crawl, if not a congested mess. Several lower stretches have been closed, and barges have run aground. Other times, towboat pilots have had to wait at narrower channels for a barge to pass through in the opposite direction before easing their own way through, snarling traffic.

One estimate put barge industry losses at $1 billion the last time the Mississippi was this low, in 1988 -- little wonder why the Army Corps has scrambled all summer to dredge, clearing shipping channels of silt and sediments while knocking down shallow spots.

To compensate, many barge owners now are carrying lighter loads, costing them more per ton to move cargo but also reducing chances of running aground as crews hustle o dredge silt and other sediments from the river to clear passages.

Copyright 2012 The Associated Press.