As of Thursday morning, there were at least 60 vessels and more than 400 barges waiting to pass through Granite City, Illinois.
The Army Corps of Engineers shut down a lock and dam there after damage to it spilled rocks into the river.
The barges waiting to move are carrying loads of cargo. But the Coast Guard says there isn't just an economic concern.
"The reason why it poses a potential security threat is because those who might seek to harm America would see this as an opportunity to see us at a vulnerable point; however, the U.S. Coast Guard is countering this by increasing our harbor patrols and increasing our security zone patrols to ensure that these vessels are kept safe as the mariners are also kept safe," said Lt. J.G. Colin Fogarty, U.S. Coast Guard.
Lock and Dam 27 is the busiest one on the Mississippi River. It moves more than 73 million tons of cargo each year.
Workers closed Lock 27 just north of St. Louis last Saturday after discovering that a protection cell - a vertical, rock-filled steel cylinder against which barges rub to help align them for proper entry into the lock - had split open, spilling into the channel tons of the rock that ultimately obstructed passage.
That damage was on an unarmored section of the protection cell that the barges don't typically make contact with because that portion often is 15 to 20 feet under water. But that part of the structure stands exposed because the river's level has been lowered dramatically by the nation's worst drought in decades, officials said.
The lingering drought also has narrowed the Mississippi - the nation's chief highway for barge traffic, leaving towboat pilots struggling to find a safe place to park their barges river as they wait out repairs. The repairs could be completed as early as Thursday, U.S. Army Corps of Engineers spokesman Mike Petersen said.
As of Wednesday morning, the stranded vessels were carrying enough cargo to fill 2,400 railcars or 23,600 large tractor-trailers
On Wednesday morning, the number of vessels forced to park there spiked by one-third over the previous 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 as workers labored to clear the spilled rock, then patch the fractured protection cell in what the Army Corps cast as a temporary fix.
"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.
"This literally is the artery of the nation, and when it shuts down there will be impacts," Army Corps Col. Chris Hall told The Associated Press while showing a reporter the damaged protection cell. Pointing to the idling barges awaiting passage nearby, with those vessels having no option of taking any detour around the repair work, he pressed: "This is a pretty important piece of terrain."
The trouble at Lock 27 is the latest barge-related headache brought on by the nagging drought.
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. That's 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.
"It's kind of bad news all around, but it's not anyone's fault but Mother Nature's," said Lynn Muench, a vice president with American Waterways Operators, a shipping industry trade group.
Martin Hettel knows the frustration. The senior manager of bulk sales for St. Louis-based AEP River Operations said that American Electric Power subsidiary has 3,250 barges and roughly 100 towboats that last year moved a combined 73 million tons of products. Much of that cargo was coal and petroleum coke, along with agricultural staples such as corn, soybeans and fertilizer.
This year, he shrugs, "it's definitely a challenge, no doubt."
"While we're confident we're able to deliver customers' freight, not just as efficiently as we would," he said, noting that the lower river levels have forced the company to reduce each of their barge's payload on average by 25 percent, about 1,500 tons. Because of the river's narrowness now, AEP's tow sizes have been cut by 30 percent.
The Associated Press contributed to this report.