A National Weather Service hydrologist testified today that it is "extremely unlikely" that the body of Nathaniel Cater entered the Chattahoochee River at the spot where police say Wayne B. Williams dumped it from a bridge last May 22.

The hydrologist, David Dingle, said his conclusion was based on a study showing that objects floating along a one-mile stretch of the river do not cross from one side to the other. Prosecutors contend that Williams dropped Cater's body into the river near its north bank, although the body was recovered from the south bank on May 24, 1981.

Dingle reached a similar conclusion about the body of Jimmy Ray Payne, with whose murder Williams is also charged, and whose body was recovered from the same area of the river on April 27, 1981.

Hydrology is the study of water and its effects on the earth.

Dingle, a witness for the defense in Williams' murder trial, admitted under cross-examination that he was not aware of a report indicating that the river's current shifted southward just upstream from the area where the two bodies were recovered.

Dingle co-authored one report about the Chattahoochee's flow characteristics last summer with a river expert from the Army Corps of Engineers. Dingle, however, was not invited to work on a second report, which revealed the current shift.

In cross-examining Dingle, a prosecutor suggested that law-enforcement agencies decided to drop him as a consultant after he granted an interview about his work to a local television reporter.

Cater, 27, and Payne, 21, were among the last of 29 young Atlanta blacks whose deaths and disappearances were investigated by a special police task force and the FBI.

In the fourth day of the defense's case, two other witnesses were called to attack the testimony of several prosecution witnesses who said they saw Williams with one or more of the 12 young blacks investigators say he killed over a 22-month period.

Charles S. Chisholm, an optometrist who said he has examined Williams' eyes for 15 years, testified that the defendant's 20/400 vision renders him legally blind without his glasses. He said Williams is so nearsighted that he "probably would have an accident within a short time" if he drove his car without wearing his glasses.

Several witnesses who said they saw Williams with one of his alleged victims either testified that Williams was not wearing glasses or they could not remember whether he was wearing glasses.

The defense also questioned a local television reporter about his interview with a woman who has testified that she saw Williams and Cater together last May.

The reporter, Paul Crawley, said Margaret Carter first told him she saw Williams and Cater together on Memorial Day. Crawley said Carter admitted that she might have been confused about the date after he reminded her that Cater's body was taken from the Chattahoochee the day before Memorial Day.