Woe is the Mississippi. A barge carrying light crude hit a bridge near Vicksburg, Mississippi, on Sunday, causing an oil spill.

But if you think that is the worst thing that's happened this winter to the river, you'd be wrong.

The middle Mississippi—the 200-mile (322-kilometer) stretch from St. Louis to Cairo, Illinois—is experiencing drought conditions unrivaled in the last 50 years. That's been the case since November.

From December to March, this part of the river is always at its lowest because extra feed from the Missouri is cut off when that river's navigation season ends. The Mississippi typically loses about three feet at St. Louis as a result.

But this winter the river has lost more depth, since spring ice melt and rains weren't forthcoming and reservoirs that help feed the river didn't get filled.

The result is that transport along the Mississippi is down dramatically. In December, total barge cargo was down more than 1,100 kilotons from December 2011. (Video: Drought 101)

Barges have had to lighten their loads considerably to avoid bottoming out. Right now barges on the middle Mississippi can only afford to sink 9 feet (2.7 meters) into the water, some only 8 feet (2.4 meters). They usually run 12 feet (3.7 meters) deep, more laden with goods to get them to market faster and cheaper.

If that doesn't sound like a lot, consider that barges lose about a hundred tons of capacity for each 6 inches (15 centimeters) less deep they can sink in the water.

According to the American Waterways Operators (AWO), in December and January alone more than $7 billion worth of goods was at risk of not reaching their destination.

"It's not like someone is going to put up a sign and say the Mississippi River is closed, but there's not very many vessels that can move in those conditions," says AWO spokesperson Ann McCulloch. (Read "Road Trip on the Northern Mississippi.)

One of the effects is that farmers on the middle Mississippi, the drought-strapped area, are paying a dollar more to ship each bushel of crops than are farmers on the lower Mississippi, who can fully load barges before sending them down the river, says Joe Kellett, deputy district engineer at the U.S. Army Corps of Engineers' St. Louis District.

For middle Miss farmers, it's more trips—so higher fuel costs—with less cargo.

Spreading the Costs of Drought

If you don't live along the waterway, likely you don't think often of the Mississippi beyond its Huckleberry Finn-fueled place in American mythology.

But you should be thinking of Big Muddy in more concrete terms. If you live in the United States and many other parts of the world, the Mississippi carries an awful lot of stuff you use every day—corn, cement, coal, and crude oil, among other things.

And the Mississippi is more central on the world stage than those who don't live beside it realize.

"Harvest to market also means Centralia, Illinois, to Tokyo," says Mike Peterson, a spokesman with the Army Corps of Engineers, which constructs and maintains the riverbed of the Mississippi, kind of like a watery Department of Transportation. He notes that Japan gets 90 percent of its livestock feed off the river.

When one of the river lock's gates broke during the 1997 harvest season, Jack Yui of Japan's Zen-Noh grain corporation sent a fax to the corps' lockmaster: "I need to know when lock and dam 27 will be repaired to know if the government will need to release the grain reserves of Japan," it read. Yui wanted a daily report.

He likely wasn't the only one. Sixty percent of farm exports for the entire U.S.—largely corn and soybeans—move along the Mississippi.

"We are blessed to have our great breadbasket and river system line up," says Dave Busse, the chief of engineering and construction for the corps' St. Louis District. "In Brazil, they grow soybeans but spend a lot to get it to the water. The Nile [and] Congo don't have much grain around them."

And choked-off agricultural exports can affect Americans too. If Kobe cattle can't get their feed, for instance, fancy burger prices would soar in the U.S.

There are plenty of other domestic implications. If road salt, shipped only in the winter months, can't shimmy northward, northern towns are hard-pressed to deal with icy streets. Fertilizer can't make it to farms for spring planting.

As the oil spill suggests, the Mississippi is carting petroleum and crude, too. Barges and tankers carried almost 48,000 barrels from the Midwest to the Gulf Coast in 2011, nearly double the amount in 2007.

It's important for other energy sources as well. If the river doesn't run at full capacity, coal from West Virginia is slow to get to St. Louis, where it fuels the power plant that fires the Anheuser-Busch factory there, one of only a handful of places in the U.S. where Budweiser gets made.

There are dozens of other power plants that pepper the river's shores that also rely on it to get coal.

How to Run a River

The Army Corps of Engineers is tasked by Congress to maintain the Mississippi as a channel that's 9 feet (2.7 meters) deep and 300 feet (91 meters) wide.

It's often a bit wider in the bends: Tugs have to tow through bends sideways, a process called planking, then let the flow turn the barges straight.

Tugs pulling rafts of 15 barges at a time—three wide and five deep—can fit through the middle Mississippi simultaneously and often do.

During winter the river is typically helped by a system of reservoirs, which allows the corps to keep the Mississippi running at its prescribed height and depth.

Water control managers make decisions on whether and how much to tap reservoirs every two hours, all day, every day.

They have to be vigilant. Water levels in the last year have dropped more than 30 feet (9 meters) from 2011's flood to current conditions.

The drought is challenging reservoirs already stretched to their limit; they didn't get enough rain to fill them enough to start with. "There's an entire ballet going on to squeeze every last drop out of the system to make sure the river stays open without impacting the other purposes of those reservoirs," says Kellett.

During a drought, the corps' annual dredging is even more important. The typical dredging season in St. Louis runs from July to December, when flow is at its lightest, to keep sediments deposited by the flow from building up.

"It's repetitive," says Busse. "The next time the water comes up, all that work disappears."

This year's dredging is more intense. "We're gathering close to twice as much as a regular year, and we're going out earlier and staying out later," says Petersen.

As a more drastic measure, the corps is in the process of lowering the river bottom at Thebes, Illinois, removing limestone and shale pinnacles that range in size from that of a bowling ball to that of a small car and that can make navigation impossible if the water goes any lower.

In the meantime, engineers have been releasing just enough extra water from reservoirs to keep navigation moving. "It was a fight of inches," says Busse.

There is 12 days-worth left of supplemental water. Busse says pinnacle removal should be completed before that water runs out. For now at least, engineering seems to be outpacing natural disaster.

Kellett notes that current low water levels are not unprecedented in the modern era. The year 1963 saw a similar low.

"The river is cyclical—in the '40s, the '60s, the '80s, the early 2000s—every other decade or so we have hit these levels of lows," he says. "What I don't know is the role that climate change is playing here."

The long-term National Weather Service forecast is for temperatures above normal, which dry out soil and evaporate more water.

"What we know is that droughts rarely occur for only one year," says Busse.

What Might the Oil Spill Do?

The lower Mississippi—the stretch from Cairo, Illinois, south to the Gulf of Mexico—had been running at normal capacity because it's fed by the Ohio River, a healthy-size tributary.

But that's the part affected by Sunday's spill.

The barge was carrying 80,000 gallons (303,000 liters) of light crude. About 7,000 gallons (26,000 liters) of oil wasn't in the tank where it should be; it's undetermined how much seeped into Big Muddy.

The Coast Guard, the river's traffic cop, closed the waterway for the cleanup.

Compared to drought effects, the spill is a shorter-term problem. The last oil spill on the Mississippi, 10,000 gallons (38,000 liters) last February, was resolved in less than a day. In 2008, 283,000 gallons (1 million liters) shut off the waterway for just six days.

This week, 5,300 feet (1615 meters) of booms helped block flow downstream toward the Gulf, and workers have skimmed 7,650 gallons (29,000 liters) of oil-and-water mixture so far.

But the cleanup has slowed traffic on the Mississippi even more.

"It will stay closed until we can safely move traffic without impeding the cleanup efforts," said chief petty officer Bobby Nash of the U.S. Coast Guard on Tuesday. The 16-mile affected stretch opened with restrictions at 7:00 p.m. on Wednesday.