Norman Hillman looked down from the pilothouse of his tugboat at three steel barges, each just 10 feet shorter than a football field, and gave the vessel a jolt of power.

The engines, backing against the powerful current, rumbled and the barges eased slowly downstream into the Mississippi River's Lock 26.

"About a foot on both sides and coming in nice and steady," crackled the voice of a crewman over the radio. Then there was a slight shudder, and the barges were in place.

It has taken Hillman 41 years on the river to master the currents and eddies that can spell disaster for an unwary pilot.

"We've all lost some," Hillman chuckled. In his day, he has seen barge convoys run aground on sandbars, smack into bridges, even disappear over dams. He has heard the taut steel cables that hold the barges together pop "like a pistol" and watched loose barges run wild downstream.

The temperamental currents are an accepted, unchanging, aspect of life on the river. But it is a new, manmade problem that the rivermen now face.

The nation's rivers are bearing the brunt of unprecedented demand on the rural transportation system, arising from the steady increase in U.S. food exports since the early 1970s.

The river system, extending from Minneapolis to New Orleans and running through the richest agricultural basin the world has ever known, has become the main thoughfare of the American grain trade.

This year, about 40 million volume of six years ago - will drain off thousands of Midwestern farms into terminals along the Mississippi, Illinois, Ohio, Missouri and Arkansas rivers and head downstream toward overseas markets.

The strains on this system are evident this summer in the record level of barge traffic, in the rising cost of shipping by barge, in the congestion at such key points as Lock 26 - where the volume of grain passing through has increased from 5.3 million tons in 1960 to 28 million tons in 1977 - and in the concern of environmentalists about the impact on the ecology of the rivers.

Upstream from the lock, convoys of 15 barges, lying low in the water from full cargoes of grain, are moored close to the river bank waiting their turn to go through the lock. Barge companies say the demand for barges is so strong this summer that grain companies have been willing to use even old coal scows to haul the commodities down the river.

At worst, the situation will reduce exports, affect the country's trade balance and result in losses for farmers, who are likely to end up footing the bill for the higher costs of shipping commodities to distant markets.

A preliminary report of a transportation task force directed by the Department of Agriculture has warned that "the [water] traffic increase cannot continue indefinitely unless channel dredging and new facilities are provided."

Congress has authorized a multibillion-dollar, nine-year reconstruction and expansion of Lock 26, and has approved continued dredging of the water channels by the Army Corps of Engineers to maintain navigation. However, the federal study noted, environmental groups have objected to the dumping of dredged material. Meanwhile, railroad and environmental interests have gone to court to try to block the funding of the new lock.

"My personal concern is that our whole agricultural transportation system is going to break down," says John W. Lambert, president of Twin City Barge and Towing Co. fo Minneapolis. "Every mode of transportation is saturated and showing signs of crisis. The railroads are short of cars and have deferred maintenance on their tracks. Heavy trucks are damaging the highway. One of these days we are going to have grain available, foreign buyers who want it, and no way of getting it to them."

Although the railroads insist that they are carrying more grain to the ports than ever before, complaints about their deteriorating service are mounting.

This summer, the daily shortage of jumbo hopper cars to carry grain and soybeans has been running around 20,000 - an amount that exceeds the shortage in 1973. That year traffic snarls developed as huge quantities of grain were shipped to the Soviet Union and floods disrupted barge traffic on the Mississippi.

Managers of small grain elevators say they are being strangled by the hopper-car shortage. Some of them charge that most of the available cars are being allocated to the "unit trains" of the big grain companies. These are the 100-car shuttles that run non-stop between inland terminals and the ports.

And as the railroad situation has worsened, grain companies located along the Illinois River increasingly have shifted from rail to barge to move commodities to the Gulf of Mexico.

These shifts have spurred the unprecedented boom on the rivers, and marked a historical reversal of sorts.

In the mid-19th century, the railroads won out over the river boats in a bitter struggle for supremacy in transportation. This helped establish the mastery of the Great Lakes and East Coast ports, and strengthened the economy of the North.

But as the railroads have faltered, the rivers have reasserted their prominence and New Orleans has emerged as the nation's dominant grain port.

Of the 10 million tons of grain and soybeans exported in June, about a third left through New Orleans, and most of it got there by barge down the Mississippi River and its tributaries.

Huge river movements are expected in the months ahead, as a result of recent administration approval of increased Russian grain buying in 1979 and 1980. The growth in river traffic can be seen from the fact that barge companies report backlogs in orders for new equipment, and that 40 percent of the 9,369 dry-cargo barges operating on the river are less than four years old.

At its northernmost point, at Minneapolis and St. Paul, the river system provides an outlet for wheat, rye, barley and oats, which are shipped by truck or rail to the big collection depots.

From there, tows with up to 15 barges full of grain head downstream, through a series of 27 locks. As these convoys move south they pick up cargoes of corn and soybeans. Traffic increases heavily where the Illinois River runs into the Mississippi north of Lock 26.

Once the tows and barges have cleared the locks, the river environment changes. After St. Louis, the river is wide open, and the convoys, called "fleets," sometimes grow to 35 or 40 barges.

St. Louis is a strategic barge "parking lot," where tows heading upstream drop off empties before going through the narrow locks with cargoes of coal, fertilizer, salt and petroleum, and other tows pick up those empty barges, which they fill with corn and soybeans on the 1,136-mile trip south.

As the barges move downstream, the traffic departments of the big grain companies in Kansas City and St. Louis plot their movements and sometimes trade cargoes back and forth to meet the needs of freighters arriving at the Gulf.

It is a complex system that rests on the skill, endurance and courage of the river bargemen.

In summer, temperatures on the decks of the steel barges reach 120 degrees. Bits of coal along the edge of a barge can send a crewman tumbling into the water as easily as if he were walking on marbles. And in winter the decks are slippery with ice.

One river man described how a fellow deckhand had fallen into the Mississippi on a trip last winter.

"When we got him back to the side of the barge, his clothes froze solid agianst the steel hull and we had to cut him out of his jacket and pants," he said.

Towboat pilots have to watch for "holes" - sudden changes in depth - that can cause underwater turbulence and snap the steel cables holding the barges together.

Most bargemen on the Mississippi River alternate a month of paid leave with a solid month on duty.

Bob Culkin, a tugboat captain for Eagle Marine Industries, which runs barge "parking lots" in St. Louis, is content with his less glamorous life.

Life on the towboats that ply the rivers, he says, is "not for a married man."

"You see the same people for 30 or 40 days, listen to the same rattles and see nothing but the inside of that same damn boat. It gets so you can't even sleep."

The magnet for the grain moving onto the rivers is the network of more than 100 barge terminals up and dow the major channels.And from one end of the system to the other, the economic role of the international grain companies is pervasive.

Multinationals such as the U.S.-based Cargill and Continental, and the foreign-based Mitsui and Bunge, own or lease the bulk of more than 60 million bushels of storage space in the 10 terminals north and south of New Orleans at which freighters load grain for foreign markets. Cargill, the agricultural collossus from Minneaplois, has a capacity of about 15 million bushels - or 25 percent of the total, according to the Department of Agriculture.

Elevators of these same companies also leapfron each other on the way up the river. And farmers eith river frontage say representatives of the multinationals frequently make offers to purchase additional sites.

Multinational grain companies also have moved aggressively into the river transportation business. Cargill spends $75 million a year to ship grain by barge. Cargill, Continental, Peavey and Archer Daniels Midland all own tows and barge fleets, and Cargill also builds barges.

Notwithstanding these signs of concentrated corporate power, a 1975 study by the Upper Mississippi Waterway Association estimated that farmers with river outlets for their grain could receive 25 cents more a bushel than those served only by rail.

Barge rates this summer have been running substantially lower than rail rates in the lower Mississippi River.

Ira Hudson, who runs an independent grain business on the Ohio River in Mound City, Ill., says he has been paying $13,110 to send a 1,500-ton grain barge to New Orleans. The comparable amount for rail is $17,759 - through Hudson says no cars are available. The barge figures translate to about 25 cents of the $2.80 a bushel that corn is selling for in New Orleans.

Twin City Barge's Lambert says barges are more efficient in their use of energy. He estimates that a tow can move 400 tons of grain a mile on a gallon of fuel, compared with only 200 tons for a locomotive.

However, a number of factors spell potential trouble. For one thing, the price of grain barges is unregulated, and prices are now rising in the "spot" market due to higher fuel costs and increased demand.

Some small investors, including drugstore owners and dentists living along the rivers, have become millionaires in the bullish barge markets of the last few years by investing their savings in barges and leasing them out like small-scale Greek shipowners.

Grain companies say that costs are being held down now by the fact tha many barges are out on old charters that are a bargain in the present market. As these charters expire and are adjusted, prices will rise.

These higher costs are sure to be passed back to farmers.

Some river grain dealers also express concern that they are wholly dependent on a market dominated by the handful of multinational grain companies that control the export elevators downstream.

Hudson said that if he could obtain rail cars he could "pay farmers an extra 10 cents a bushel." This is because prices for corn and soybeans in the East and Southeast - the centers of the U.S. broiler industry - are running above prices at the Gulf. However, Hudson has been unable to obtain hoppers cars that go to these domestic destinations.

Nature also imposes limits on future growth in barge traffic. During the winter months, ice on the Upper Mississippi, Illinois and Ohio rivers hampers operations, leaving grain shippers "at the mercy of the railroads," as one transportation expert put it.

The Great Lakes also have built-in limits to their ability to serve as a relief valve. Deep-sea vessels larger than 20,000 tons have trouble passing through the Welland Canal connecting Lakes Erie and Ontario, and the lakes are frozen from December through March. And the Missouri River, which cuts through the heart of the farm belt, is so shallow and treacherous that it is risky to take convoys of more than three lightly loaded barges down it.

Another concern to barge companies is the impact of a new federal toll on river traffic. The toll, the first in history, was approved by Congress as the price tag for authorizing reconstruction of Lock 26. It will be in the form of a 4-cent-a-gallon tax on barge diesel fuel, to rise later to 12 cents.

The American Waterway Operators has left no doubt who will foof the bill.

"Barge companies don't pay user taxes anymore than beer companies pay excise taxes," said the AWO. "Only consumers pay."

This is another way in which Americans will continue to feel the impact of rising food exports - a development resulting from such distant economic events as the Soviet Union's need for more U.S. corn and soybeans to feed its hogs and poultry.

There are still many parts of the Midwest rivers that look as untouched as they did when the first French explorers navigated them. These are places where the muddy waters still slide at three knots past uninhabited river banks lined with nothing more than windswept trees.

From such perspectives, the rivers are still a pure natural resource.

But the mounting economic demands of the grain trade on the rivers seem to assure a long struggle over their destiny. CAPTION: Illustration, no caption, Page design by Richard Furno - The Washington Post; Map, no caption; Picture, no caption