Joe Serbus farms outside the town of Franklin, Minn., near the winding Minnesota River. The water levels have been low for a while now, but it's the other river — across the state — that he's worried about.

"The Minnesota [River] really started dropping in June. But just a week ago, we did get an email about that bottlenecking [on the Mississippi River] was starting below St. Louis," said Serbus, chair of the Minnesota Soybean Research & Promotion Council.

And the bottleneck on the Mighty Mississippi comes just as fall harvest in the Upper Midwest is at full tilt.

Crops from Minnesota fields are transported to grain terminals along the Mississippi and, eventually, shipped down on barges to New Orleans, where they're loaded onto vessels headed overseas. About 60% of America's corn and soybean exports travel at some point down the Mississippi River to coastal ports.

"It's game-time for agriculture. It's harvest time," said Mike Steenhoek, executive director of the Soy Transportation Coalition, from his office in suburban Des Moines. "The river operates as a whole linear system, and you're only as strong as your weakest link."

The system has dramatically slowed this fall as the normally wide, deep portion of the river down south has lost volume, running boats aground.

After a towboat became lodged near Vicksburg, Miss., earlier this month, barge traffic backed up to a queue of some 1,700 vessels. The water level in New Orleans measures at about 2.5 feet — roughly half its usual depth this time of year. It's so low in Baton Rouge, La., that a wrecked 19th-century ship has been exposed.

While traffic commenced again this week, the forecast shows no signs of meaningful rainfall. Soon, winter will force the closure of the riverway from Quincy, Ill., to the Twin Cities.

What happens downstream affects the whole river chain, including back on Serbus' farm.

"It's getting those beans coming down the river and fertilizer up the river," said Serbus, of the river's importance.

At the port in Winona, Minn., barge traffic dropped off in September to only 50 vessels — the lowest total in a decade. This week, trucks loaded with grain stood in a queue along the river road, waiting to offload a summer's crop.

"The lower Mississippi is kind of a mess," said Dan Nisbit, president of CD Terminal in Winona. "Extremely low waters down there. And they're having to have draft restrictions."

Water levels on the lower Mississippi often allow for northbound boats to have up to 14 feet of hull beneath the surface, Nisbit said. But new cargo restrictions are limiting boats to a 9 feet or shallower depth along many sections of the river.

Trucks carrying corn and soybeans in Minnesota have lined up this fall to fill terminals in St. Paul to Red Wing to Winona. But with barge traffic slowing, rates to rent a vessel are jumping as grain prices soften.

A barge headed down the river purchased in the Twin Cities costs over $100 per ton, according to the latest USDA survey. Last October, the cost was less than $35 per ton.

The grain buyers are reluctant to share how the barge traffic is or isn't impeding operations. Chicago-based ADM (formerly Archer-Daniels-Midland) referred all questions to trade associations and did not respond to a second request for comment.

In an emailed statement, Inver Grove Heights-based CHS Inc. acknowledged it is keeping a close eye on the "situation" on the Mississippi.

"We will continue to leverage our integrated and nimble supply chains, including our deep-water ports, to serve our customers and help meet the needs of the global supply," its statement said.

Barges carry as many bushels of grain as 35 train cars or 134 semitrailer trucks, according to the U.S. Army Corps of Engineers. Climate models predict a wetter Upper Midwest in the long run. But for the second year in a row it's drought — largely due to a lack of moisture in the ground after months of unseasonably low rainfall — that has greeted farmers and shippers in the northland.

"When we are in drought, and the ground is very dry, the groundwater level is low and being depleted, like it is now," said Dan Fasching, Mississippi River regulator with the U.S. Army Corps of Engineers in St. Paul.

Unlike the upper Mississippi, which has a series of locks and dams to artificially pool water depths, the lower stretch of the river — from St. Louis through Memphis to New Orleans — is less industrialized, given its historic, natural depths.

"Down south of St. Louis," Fasching said, "they usually have enough river flow that a navigation dam isn't needed."

Dredging this month to deepen water levels near Stack Island, 200 miles north of New Orleans, restarted traffic. Such ripple effects are welcome news upriver, where autumn harvest awaits vessels.

According to the U.S. Department of Agriculture, Minnesota farmers on Monday reported harvesting 14% of the corn and 63% of the soybean crop. Many farmers fear the price on stores of fertilizer, which would normally be replenished by upbound cargo vessels, could increase.

"You can see that'll be an obstacle, if it's not there already," Serbus said.

One barge takes nearly four weeks to reach New Orleans from the Twin Cities, says Steenhoek of the Soy Transportation Coalition. Soon, the last barges will shove south, and it'll be months before higher temperatures bring them up again.