JOHN YANG: A warm fall and expanding extreme drought conditions have helped water levels along the.

Mississippi river drop to record lows.

Special correspondent Megan Thompson reports from Missouri on what conditions on this vital commercial route mean for farmers who rely on to get their crops to market.

MEGAN THOMPSON: October is one of the busiest times of year for Iowa farmer Robb Ewoldt.

His crops are ready for harvest and he needs to work fast to take advantage of good weather like today.

ROBB EWOLDT, Iowa Farmer: We are currently harvesting soybeans and we are hauling them directly down to the river terminals for export.

MEGAN THOMPSON: Ewolt sells his soybeans for export because that's where he gets the best price.

And lucky for him, his farm in eastern Iowa is just a few miles from the nation's most important grain shipping corridor, flowing 2,300 miles through the heart of the country.

The Mississippi is the nation's second longest river.

It's also a superhighway for American agricultural products.

Around 60 percent of the grain exported from the U.S. is sent down the river by barge to the Gulf Coast.

For some farmers in eastern Iowa, like Ewoldt and his friend Joe Dierickx access to the Mississippi is critical.

ROBB EWOLDT: It's a very important it.

I mean, it's a vital part of our operation.

JOE DIERICKX, Iowa Farmer: The Mississippi River is really our lifeline for exporting around the world.

MEGAN THOMPSON: Usually, Ewoldt and Derek spend these harvest weeks driving nearly non-stop back and forth from their fields to grain elevators on the river.

But this year, the routine was upended.

Ewoldt semi-truck got stuck in two hour lines and Dierickx was told not to show up at all.

JOE DIERICKX: The elevator told me that they were full.

They didn't have a barge that they could dump any more beans into and they were going to close at 02:00.

MEGAN THOMPSON: Closing unexpectedly because the giant barges that are the main mode of transportation are having a hard time getting up the river.

In October, water levels drop to the lowest ever recorded.

The low levels are exposing the riverbed and rocks, slowing down the thousands of barges that operate here.

BRANDON PHILLIPS, Towboat Captain: I've dealt with low water before, but not this low.

MEGAN THOMPSON: Brandon Phillips is a towboat captain for the American River Transportation Company, or ARTCo, a subsidiary of agriculture giant ADM. With 2,000 barges on the Mississippi, it's one of the largest operators in the US.

BRANDON PHILLIPS: Hey, I'll be crossing river beyond the one for you.

MEGAN THOMPSON: When we met Phillips in October, he'd just returned to St. Louis after 31 days straight on the river.

BRANDON PHILLIPS: These are just our typical hopper barges.

We'll tie them together with steel cables, and then we'll hook them up the boat and push them downriver.

MEGAN THOMPSON: The barges fill up at river terminals like this one in St. Louis, where grain is trucked in to be weighed, then dumped and stored.

A chute delivers the grain into the barges, which are then tied together into huge fleets.

Chad Hart is an agricultural economist at Iowa State University.

CHAD HART, Iowa State University: As we're thinking about the barge system, why do we rely on it so much?

It is because it is the most cost effective way for us to move our crops up and down a river.

MEGAN THOMPSON: According to a study by the Texas A&M Transportation Institute, one barge can carry the same amount of dry cargo as 16 train cars or 70 trucks.

Barges have a much smaller carbon footprint, too.

CHAD HART: So it's an incredibly large amount of grain that you're able to move quite easily and effectively down the river when we have full capacity along the Mississippi.

MEGAN THOMPSON: But this year, the barges are only carrying about two-thirds of what they normally would.

The lighter loads help them float higher in the water to avoid getting stuck on the river bottom.

The low water also means the river is narrower.

In a normal year, ARTCo could lash together 46 barges of grain for a trip south.

This year, the most they can fit is 25.

BRANDON PHILLIPS: We're not loading the barges as heavy, so we have to make more trips.

So you have more boats down there taking fewer barges, doing more laps.

MEGAN THOMPSON: And those laps are taking longer, in part because the shallow river is harder to navigate.

BRANDON PHILLIPS: You've got these two red triangles that tells me that it's gotten shallow on the backside there.

That tells you to take warning.

MEGAN THOMPSON: Even with the warnings, boats can run aground, which could mean jamming up river traffic and turning a trip from St. Louis to New Orleans that normally takes five days into a nine-day voyage.

BRANDON PHILLIPS: It's a lot of work.

It takes a lot of planning, takes a lot of communication with other vessels because you have a lot more traffic to deal with.

It's nerve racking.

Even being as experienced as I am, I don't have control over what other people do.

I don't necessarily have control over what Mother Nature does.

MEGAN THOMPSON: The barge problems caused prices for shipping on the Mississippi to spike this fall.

Right when farmers were trying to sell their grain.

To make up for the increased freight costs, grain buyers lowered the price they offered farmers.

JOE DIERICKX: They pay us less.

That cost is pushed back to the farmer.

MEGAN THOMPSON: You can't turn around and pass that cost on to somebody else.

JOE DIERICKX: I would love to pass that on to somebody, but I can't.

MEGAN THOMPSON: Joe Dierickx says he lost about $15,000 on his soybeans compared to last year.

Rob Ewolt is down around $25,000.

ROBB EWOLDT: I don't think anybody wants to take a $25,000 hit to their paycheck.

MEGAN THOMPSON: The issues on the Mississippi are adding to another problem.

Over the last few years, high transportation costs have begun to hamper international trade, contributing to higher prices for American goods, says economist Chad Hart.

CHAD HART: So it makes us less competitive in the international marketplace.

Over the past couple of years, Argentina and Brazil have been able to erode our market share in certain key markets, especially as we look into China.

MEGAN THOMPSON: That the barges are still able to reach these markets at all is thanks mostly to the U.S. Army Corps of Engineers.

Since mid-July, the Corps has been keeping the channel open by dredging, sucking out massive amounts of from the bottom of the river.

LOU DELL'ORCO, U.S. Army Corps of Engineers: What we see is our dust pan.

So our dust pan is our vacuum cleaner.

35 feet wide.

MEGAN THOMPSON: Lou Dell'Orco is the chief of operations for the Cores St. Louis District.

He oversees this massive boat called the Dredge Potter working near Scott City, Missouri.

The boat moves this machine a huge vacuum around the bottom of the river.

LOU DELL'ORCO: Once the dredge sucks it up through the vacuum cleaner, it runs through a pipe.

The pipe goes from the front of the dredge all the way out there and there it's deposited outside of the channel.

In an average day, we can move about 50,000 cubic yards or enough sediment to fill an Olympic sized swimming pool every hour.

MEGAN THOMPSON: A rotating crew of 35 people live on the boat for around three weeks at a time, running the dredge 24 hours a day.

MAN: This screen here, it shows you where we're dredging at, where we're digging.

MEGAN THOMPSON: It's one of eight Army Corps dredging boats working on the Mississippi right now.

It's hard work, made harder by the fact that this vessel was built in 1932 and requires a lot of maintenance.

LOU DELL'ORCO: They're maintaining a 91 year old vessel and there's challenges abound.

And the team keeps it together.

Everybody's dedicated to the mission, which is maintaining the channel which supports the nation's economy.

MEGAN THOMPSON: Dell'Orco's team could be out here until winter, when everyone hopes snow and rain will finally arrive and replenish the river, returning water levels and business back to normal.

For PBS News Weekend, I'm Megan Thompson in Scott City, Missouri.