Radar beamed toward earth from a space shuttle experiment last year has revealed ancient, unknown river systems as large as the Nile buried beneath the sands of southwestern Egypt, according to a report released today.

Legends about the "Bahr-bela-ma," the "great rivers without water" beneath the sands of the Sahara Desert have been told since ancient times, and the legends have sent explorers into this area, the driest and most featureless expanse of sand on earth.

But the great river valleys remained hidden beneath yards of sand for thousands of years, with Stone Age man being the last to see the great valleys, stream channels and broad flood plains until the radar images were captured by the shuttle on its second flight, a year ago this month.

The team of eight scientists who made the report were surprised by the startlingly different terrain seen under the sand, a discovery which was made in a random radar sweep during an aborted experiment on the trouble-ridden mission.

"The main point, the really exciting aspect of this discovery is that there is no trace on the surface of the extensive river topography that exists underneath the . . . flat sands and long trains of marching dunes. The radar penetrated through to give us an entirely different view of this area than we ever had before," said John F. McCauley of the U.S. Geological Survey, a leader of the team that conducted the experiments, and reported in the current issue of Science magazine.

The scientists have since visited some of the sites to verify what the radar images show.

A huge area in the core of the Sahara Desert near the borders of Egypt, Sudan and Libya once had a vastly different climate and several great rivers inhabited by men and animals. Tools and other human artifacts have been found in some spots of the area before the radar discoveries, and more have been found now because the radar pictures guided geologists to areas where ancient rivers lay beneath the sand.

McCauley said the pictures were possible because the area is extremely dry. The region is hostile to almost all living things and rain is believed to occur only at intervals of 40 years or more. But when radar is beamed down on such dry area, it penetrates the sand, down perhaps 15 feet or more as opposed to the usual penetration of wet earth of only a few inches.

In this area of the Sahara, bedrock a few feet under the sand bounced back bright images, while the deeper river valleys showed up as great dark veins.

By laying the images alongside other satellite images of the Sahara, McCauley said, "it's like looking at an X-ray" because the radar penetrates so deeply. Though radar images of other parts of the Earth were also taken and are still being studied, he said, "those show all kinds of other interesting things, but only in the Sahara were we able to look through and use radar as a time machine."

The climate of the area dried suddenly and drastically from savannah or grassland to desert about 2 million years ago when the rest of the world became cooler, heading into an Ice Age. Scientists have now been able to identify three periods of a few thousand years each since that time in which the area has received enough rain to grow plants and lure back men and animals.

The periods were about 200,000 years ago, 60,000 years ago and 10,000 years ago, with the last period ending and the area drying once more just before the rise of the great Egyptian civilization. Each "rainy" period was inhabited by a different variety of man, beginning with man's immediate ancestor, homo erectus, then Neanderthal man, and finally modern man.

McCauley said the area is now so dry that on one of his expeditions there, the scientists found World War II British Army encampments with cardboard boxes, cigarette papers and truck tracks still fresh and untouched by rain.

Without more extensive pictures, McCauley said it is still impossible to tell how the buried river valleys connect to present-day river systems, but he said it is unlikely that they link to the Nile. "The trending of the rivers is to the south and west, the opposite of the present day movement," he said. "It is possible they all joined up to one large basin of interior drainage as large as the Caspian Sea is now."

The results of the radar experiment were reported by scientists from the U.S. Geological Survey in Flagstaff, Ariz.; the University of Arizona; the Egyptian Geological Survey and Mining Authority in Cairo, and the National Aeronautics and Space Administration's Jet Propulsion Laboratory, in Pasadena, Calif.

The work was funded by NASA to test the viewing power of radar from space. Another experiment similar to the first will be carried on another shuttle mission in mid-1984, McCauley said.

The second shuttle flight, scheduled for five days in space, was cut short at two days because of a faulty fuel cell that might have exploded and had to be turned off, leaving insufficient fuel cell backup to continue. Numerous experiments were canceled or cut short by the shortened mission.