A water expert and a diver testified today that it was unlikely that Nathaniel Cater's body was thrown into the Chattahoochee River from the spot where prosecutors say Wayne B. Williams dropped it.
David Dingle, a National Weather Service hydrologist who used inflatable life-size dummies and oranges to track the river's flow, said it was ''extremely unlikely'' that a body dumped into the west side of the river would end up on the other side.
Later, Michael Gurley, a diver with the Fulton County Fire Department's rescue squad, testified it was ''out of the bounds of likelihood'' that Mr. Cater's body would have surfaced where and when it did if it had been dumped a mile upstream two days earlier, as prosecutors contend.
Mr. Williams has pleaded not guilty in the slayings Mr. Cater, 27 years old, and Jimmy Ray Payne, 21, two of 28 young blacks killed here over a 22-month period. West Side of Bridge in Question
Prosecutors contend that Mr. Cater's body was thrown into the river from the west side of the Jackson Parkway bridge, where Mr. Williams was seen by a police surveillance team in the predawn hours of May 22, 1981. Mr. Cater's body was found near the east bank of the river May 24, not far from the spot where Mr. Payne's body was found a month earlier.
Mr. Gurley testified that he and another diver were asked to search the river May 23 but were not told what they were looking for. He said they recovered ''a safe, a Thompson machine gun, a Checkmaster machine and assorted jewelry.'' ''If it would have stayed under the bridge, we would have run into it,'' he said, referring to Mr. Cater's body. Mr. Gurley said he talked to prosecutors after learning that Mr. Cater's body was found a mile downstream from the bridge. 'Distance Not Likely'
''I told them that was, in my opinion, far to travel in two days - that was ou t of the bounds of likelihood,'' h e said. Mr. Gurley said he would expect a body to travel no more than 200 feet in two days. In cross ex amination, however, Mr. Gurley conceded that his experiences h ave been with victims of drownings, not people who had been killed b efore they were thrown into the water.
He also conceded that he had searched 20 to 25 minutes, which he called ''a short time for a body recovery search.'' He said visibility in the water was poor.
Earlier today, Mr. Dingle testified that he would not expect a body dumped on one side of the river to float to the other side. Mr. Dingle's testimony was allowed over the objections of prosecutors, who argued that the inflatable dummies he had used to test the river's current could not accurately simulate the movements of a human body in the river.
Mr. Dingle said his experiments were ''original, brand new research'' and were done with ''very high scientific standards.''