By PAUL RECER
AP Science Writer

WASHINGTON -- A series of mounds built by moving tons of dirt and gravel may be the oldest known complex of earthern structures built by humans in North America, researchers say in a study published today.

The low mounds on a river plain in what is now Louisiana were built about 5,400 years ago by people who found food in nearby rivers and forests and occupied the site over hundreds of years, researchers say.

The Watson Brake earthworks is ''the earliest such human construction so far recognized in the New World,'' Joe W. Saunders of Northeast Louisiana University and his co-authors said in the study published in the journal Science.

''It is the oldest mound complex, in my opinion,'' said Saunders, but he acknowledged that there may be remains of other types of structures in the Americas that he does not know about.

He said there is an older mound in Canada, but that is a single burial mound and was not a complex of dwelling mounds like those in Louisiana. Other mound complexes in North America, said Saunders, are younger than Watson Brake.

Researchers in 1986 reported discovery in Stanislaus National Forest 150 miles east of San Francisco of a clay floor that was dated at about 10,000 years old. Remains of what may have been another structure near Hells Gap, Wyo., have been dated to about 8,000 years.

Saunders said the people who built the mounds in Louisiana were seasonal hunters and gatherers who ate a lot of fish while living near a river for only a few months at a time.

Bones of catfish, drum and suckers, all common fish found even today in the rivers, were unearthed in the mounds. There was also evidence that the people feasted on turtles, mussels, aquatic snails and small animals.

It has long been believed that such hunters and gatherers lacked the organizational skills to build mounds, which are large humps that required digging up and moving huge amounts of earth, Saunders said.

''These mounds contain hundreds of tons of dirt and gravel,'' said Saunders. He said there are 11 mounds in a rough circle 280 yards across. One of the mounds is over 20 feet high. The rest are 3 to 14 feet high. Many are connected by excavated ridges.

It was, said Saunders, a major project for people who had to move all of the material by hand.

Chemical dating of materials showed that construction of the mounds started about 5,400 years ago, making them about 1,900 years older than mounds found in Florida and elsewhere in Louisiana.

Just what the primitive people used the mounds for is still not known, said Saunders. He said it is unlikely the mounds were important for defense and they are too far from the river to be used as a refuge from floods.

Other mounds have been found in the valleys of the Mississippi and Ohio rivers, but those were built by a later, more advanced people who had discovered how to make clay pots. Those mounds include pottery shards that are missing from the Louisiana mounds.

''The Watson Brake mounds are pre-ceramic,'' said Saunders. This means that the people living there had no vessels in which to cook.

Instead, he said, they apparently heated rocks and then dashed them with water to make steam.

Copyright 1997 The Log Cabin Democrat