A police rookie staked out under a Chattahoochee River bridge last May 22 testified today that he heard a loud splash like a body hitting the water, then looked up and saw lights from a Chevrolet station wagon.

The lights from the car appeared directly above the spot where he saw big waves churning the tranquil river seconds after the splash, Officer Robert Campbell testified. Moments later, lawmen halted the station wagon to question Wayne B. Williams.

Two days later, the body of Nathaniel Cater, 27, washed ashore downstream, near where the body of Jimmy Ray Payne, 21, had been found a month earlier.

Williams is on trial amidst heavy security for their murders, two in a string of killings of 28 young blacks found murdered here in two years. It was a day of damaging testimony for the defendant.

The splash sounded like a "solid bam," much louder than the noise made by beavers scurrying about his hiding place on the riverbank, said Campbell, 27.

He appeared more afraid of beavers than of Williams' sneering defense lawyer, who suggested that the officer, a former lifeguard, should have dived into the river to fetch what he believed to be a body.

"There could have been all kinds of animals and snakes in the water," said Campbell. "I wasn't about to jump in the river under any circumstances."

Campbell was alerted to cars crossing the Jackson Parkway bridge by the sound of a metal cable expanding, but he testified that he never heard Williams' car approach.

Prosecution witnesses said Williams' car apparently crept across with its lights off at less than 5 mph, and only faster-moving cars made the cable pop.

Seconds after the splash, Campbell said, he radioed fellow recruit Freddie Jacobs and asked, "Freddie, is there a car on the bridge?"

"Yes, I see it," Jacobs replied. "It's heading my way."

FBI agent Greg Gilliland and another officer took their cues, eased unmarked cars out of hiding, and pulled Williams over. He was polite and cooperative.

" 'This is about those boys, isn't it?' " Gilliland quoted Williams as saying. The FBI agent said Williams wore dark slacks, a blue sweater and a black baseball cap, and appeared "very nervous and somewhat excited."

In his car, police spotted two grocery bags, several cardboard boxes, a 24-inch nylon rope, a flashlight, suede gloves, a carpet full of dog hairs from a family German shepherd and a pair of dark slacks.

Williams said he wore the slacks to play basketball, Gilliland testified. The bags were filled with clothes, which Gilliland said Williams identified as his basketball clothes and his mother's clothes.

Interviewed by Gilliland and another FBI agent, Williams, a self-styled talent scout, was quoted as saying that he had just tried to telephone two sisters he was scheduled to interview later that day. He said he didn't want to get lost on the way to their apartment, so he had pulled off at a pay telephone up the road to get directions. It was a bum number, he said.

Gilliland said Williams denied stopping or driving slowly across the bridge, or throwing anything off. In response to a question from Gilliland, Williams denied that he was a homosexual, the FBI agent testified.

Before the sun rose May 22, police tried to recreate the splash for officer Campbell. A 216-pound police officer testified that he stood atop the railing and heaved off concrete blocks found nearby.

Splashes made by the first two blocks did not sound loud enough to Campbell, who stood below. Those blocks weighed about 60 and 100 pounds, according to the officer who threw them off. An amateur weightlifter, he estimated their weight.

A third was thrown off. Campbell said it sounded almost as loud as the splash he had heard earlier. It weighed, according to the officer who threw it off, about 130 pounds--16 pounds less than Nathaniel Cater.