Farming Argentina's pampas - by boat. Some of Argentina's most productive farm lands are under water. The flooding, primarily due to damaging environmental policies, has been compounded by heavy rains since 1983. It is a soggy disaster for hundreds of farmers and businesses in a region that normally produces more than 6 percent of the nation's agricultural exports.

With paisley silk ascot and gaucho pants billowing in the wind, Alastair Henderson is the picture of the Argentine gentleman farmer on his century-old ancestral estancia, or cattle ranch. But oddly Mr. Henderson is puttering across the pampas - the South American Great Plains - in a tippy launch with a gaucho ranch hand at the tiller.

Farmers have taken to herding cattle by boat these days in this northwestern corner of Buenos Aires Province. A bizarre inland sea has swallowed a chunk of land larger than the state of Massachusetts.

``When we saw the ducks and moor hens swimming across the front gate, we knew the flood was here,'' Henderson says.

The semi-accidental, seminatural sea is a classic example of environmental ignorance and neglect, say agricultural and environmental experts. It is already an economic disaster for the region, which provides agriculture-reliant Argentina with 6 to 10 percent of its beef and grain production.

The flooding began slowly in 1979 when a swampy, slow-moving river in the provinces of Cordoba and San Luis was reengineered in order to create new farmland. Done without multiregional discussion, study, or consent, the rechanneling of the R'io Quinto soon began dumping water onto the Buenos Aires pampas, says Oscar Dom'inguez, a scientist with the federal government's National Institute of Agricultural Technology (INTA).

The pampas were too flat to drain the water fast enough and flooding started in low-lying areas. Between 1981 and 1983, as one area became flooded, local officials and farmers cut a canal through high ground to send the water further south and east toward the Atlantic 300 miles away. A series of areas were flooded and canalized in the same way, passing flood waters to neighboring regions without their consent, Dr. Dom'inguez says.

The man-made flooding has been compounded since 1983 by heavy rains. The month of March 1987 alone had as much rain as had recently fallen in a full year. It was a foreseeable phenomenon, says Dom'inguez, who has studied historic rain charts and 15 years of NASA satellite photos of the region.

``It's a cycle that happens every 60 to 80 years, so when suddenly a humid cycle comes the farmers can't believe it,'' says Stella Carballo, also with INTA.

Regional Indian place names - like Pehuajo, deep lake, and Trenque Lauquen, round lagoon - should have been historic red flags that the region is subject to cycles of heavy rain. Henderson himself remembers his grandfather's stories of flooding earlier this century.

But, says Ms. Carballo, the past floods didn't last as long as the current one is expected to last. She says it could take five years for evaporation and slow drainage toward the sea to dry out the Pehuajo area.

INTA had warned of the rains as early as 1980, but ``no one believed us,'' says Ms. Carballo. So those responsible for redirecting the river and the subsequent canal cuts didn't take into account what their actions would do in combination with heavy rains, she says.

Indeed, the enormous physical dimensions of the problem seem unbelievable. From the air, as far as the eye can see, the region is a shallow sea of muddy brown water. Fence posts and telephone poles are still visible. Islands of high pasture bud green during the spring, and farmhouses are connected to dry land by muddy threads of roads.

Some 6 million acres are actually under water, and an area half again as large cannot be cultivated because it has been isolated by flood water. The total area out of production has represented between 6 percent and 10 percent of Argentina's agricultural exports, say agricultural officials. Exports account for as much as 80 percent of the foreign exchange earnings Argentina needs to pay its $54 billion foreign debt.

It is a soggy disaster for hundreds of farmers and thousands of businesses in the region. Henderson has dumped a month's worth of milk production and a whole crop of grain sorghum because delivery trucks are too heavy for the soggy rural roads. A rural general store owner says freight charges are higher because delivery trucks must go 111 miles around flood waters instead of the usual 43 miles.

So far, the loss of land has not affected the national economy. Statistics show Argentina had been selling less grain anyway because of the glutted world grain market and because cattle herds are at a point in the economic cycle where they are being rebuilt rather than sold off.

Ms. Carballo says Argentina has been able to maintain export levels because other regions have compensated for the loss with increased production. ``But if it were trying to increase its production it couldn't'' because of the floods, she says.

The federal and provincial governments are too financially strapped to offer much assistance in the problem. Further, says Dante Quinterno, publisher of Dinamica Rural, a monthly agriculture magazine, the problem is hidden from the urban Argentine majority because of its sheer distance from population centers and because the current market situation has kept its potential economic effects from being felt.

``It's 8 percent of export of grain and beef, and the government doesn't see it as a loss,'' says Mr. Quinterno. ``True, 8 percent isn't all the [national] production. But Argentina isn't in an economic position to say we can spare 8 percent ... we can't afford to lose anything.''

But there is no end in sight for the problem as the R'io Quinto continues to drain into the area.

Man-made problems like this are not characteristic of just the third world, says Norton Strommen, chief meteorologist with the US Department of Agriculture. He follows weather conditions and their relation to crop yield around the globe.

The Great Salt Lake and the Great Lakes have risen and fallen with disastrous effects in past decades, he notes. The reasons were both man-made and natural, he says.