Roy E. Oltman, 65, an internationally know hydrologist with the U.S. Geological Survey, died Wednesday at Fairfax Hospital after a long illness.

With the survey for more than 30 years, he had been named Engineer of the Year in February by the Virginia Society of Professional Engineers for his achievements in hydrology and hydraulics.

Among Mr. Oltman's major accomplishments was leading a U.S. Geological Survey team on the first hydrological survey of the Amazon River in Brazil in 1963.

For the first time, the team established scientific measurements of the width, flow and physical and chemical characteristics of the world's greatest river in the world's densest jungle.

The group found the rate of flow of the Amazon was five times that of the Congo River and 12 times greater than the Mississippi's. They also found that in places the Amazon was more than 300 feet deep; it dumped 3,400 million gallons of water into the South Atlantic every minute; it drained an area exceeding 2.3 million-square-miles.

"This is the king of all rivers. Every measurement we made shattered previous estimates," Mr. Oltman said after the team's return.

The techniques developed by the team on that expedition are now used routinely to study major river flows in this country.

Born in Minneapolis, Mr. Oltman received a degree in civil engineering from the University of Minnesota. He joined the Geological Survey as a field assistant in Minnesota in 1934.

After serving with the Navy in World War II, he rejoined the Survey in Nebraska and directed the engineering studies of the 1951 floods in Kansas.

In 1955, Mr. Oltman was transferred to Washington, where he organized the training, recruitment and management operations of the Survey's water resources division.

In 1960, he headed a Survey group that assisted the Senate Select Committee in preparing a landmark report that became the basis for nationwide water legislation.

From 1965 to 1967, he was senior member of the Interior Department's new Office of Water Resources Research. He evaluated hundreds of proposed research projects and coordinated 51 water research centers around the country.

Mr. Oltman returned to the Geological Survey in 1967 as assistant division chief for research and technical coordination. He retired in 1972, but continued to take on short-term assignments to Brazil for the U.S. government.

He held many honors, including the Interior Departments' Distinguished Service Award and the Virginia Society of Professional Engineers' Distinguished Service Award.

Mr. Oltman had served as treasurer of the International Association of Hydrological Sciences, executive director of the International Water Resources Association and executive director of the Virginia Society of Professional Engineers.

He was a member of the Americna Society of Civil Engineers, the American Association for the Advancement of Science, the American Geophysical Union, the American Water Works Association, the American Society of Photogrammetry and the National Society of Professional Engineers.

He is survived by his wife, Evelyn, and a son, Mark, both of the home in Falls Church; two daughters, Mary Bishop, of Chicago, and Maria Van der Sluys, of Shelby, Mich., and a sister, Dr. Jane Elizabeth Oltman, of Newton, Conn.