Scherr, 31, did not testify directly about what happened when Rough Surface was killed Aug. 2, 1980.
Scherr's lawyer gave a brief description of events surrounding the death of the 18-year-old woman. Circuit Judge Jack Von Wald then asked Scherr if the lawyer's account was accurate.
"Yes, it was," Scherr replied.
The plea came during a 30-minute hearing the day his trial on a first-degree murder charge was to begin.
Some members of the Rough Surface family were disappointed because they had hoped to hear Scherr admit that he had killed the young woman.
The state dropped the murder charge, which would have carried a mandatory life prison sentence, in exchange for Scherr's plea. A rape charge had been dropped earlier because the time to prosecute it had expired.
Under South Dakota law, the maximum sentence for first-degree manslaughter is life in prison.
Scherr, who has been free on bail, will be sentenced May 22. When the judge ordered that he be held in jail pending his sentencing, members of the slain woman's family applauded.
Walworth County State's Attorney Dan Todd said the state will recommend Scherr be sentenced to 100 years. The defense can argue for a lesser sentence.
The Rough Surface family appeared divided about the plea agreement.
Clara Oxford of Mobridge, a sister of the slain woman, said she was satisfied.
"I feel sorry for him," Oxford said of Scherr, "because he was so young, and hedidn't know what he was doing. He was so drunk."
However, when Todd explained the plea bargain to family members just before Tuesday's hearing, Mark White Bull said the Rough Surface family did not support the agreement.
"The family doesn't like the proposed agreement because Nick Scherr could possibly walk in a couple of years," said White Bull, a friend of the Rough Surface family.
If Scherr gets 100 years, he could be eligible for parole in about 13 years. But people convicted of homicides rarely are paroled when first eligible, officials said.
The case has caused racial tension in north central South Dakota, and White Bull said the plea bargain again showed the disparity between the races. Scherr is white.
Everyone entering the Walworth County Courthouse on Tuesday had to pass through a metal detector. Law officers were stationed throughout the building.
Rough Surface disappeared after an August 1980 trip to Mobridge, just across the Missouri River from the Standing Rock Sioux Reservation. She was found nine months later, when a rancher discovered her decaying remains in an evaporating bay in the Missouri River.
The case remained a mystery until Scherr's cousin, James E. Stroh II of Eagle River, Wis., confessed last fall that he had helped Scherr beat, rape and fatally shoot Rough Surface.
Stroh will be allowed to plead guilty to second-degree manslaughter and aggravated assault because he had pledged to testify against his cousin.
Stroh has said he told his wife before they were married that he had helped kill Rough Surface. Because of a bitter divorce, someone in his wife's family apparently decided to tell authorities, according to court records.
At a preliminary hearing, Stroh said his family was on vacation and had stopped for a few days in Mobridge when he, then 15, and his 16-year-old cousin, Scherr, met Rough Surface.
After the three left a party in Scherr's truck, the woman hit him, Stroh said. That angered Scherr, who stopped the pickup truck, dragged Rough Surface outside and raped her, Stroh said.
Stroh said he also raped the woman at the insistence of his cousin. He said they shot her, chained her body to the truck, dragged it to the river and dumped it.
At Tuesday's hearing, defense lawyer Reed Rasmussen of Aberdeen gave a brief account of what happened. He said the killing occurred after a night of drinking.
"She was shot and killed," Rasmussen said. "Mr. Stroh and Mr. Scherr transported the body to the river and put it in the river to hide it."