Four-year-old London and her mother, 34-year-old Brenda Nalleli DeDios, were sitting by the banks of the Provo River in the US state of Utah on Monday when the little girl was swept into the raging current.

Brenda and a bystander, Sean Thayne, 30, jumped into the river after London as she was swept downstream by the river’s whitewater. At least four other adults also jumped in the water to help rescue the girl.

Brenda and Sean – who did not know the mother or her daughter – were unable to escape from the water and were pulled from the river nearly 5km from where they jumped in.

Both were in a critical condition when they were pulled from the water, and were transported to Utah Valley Hospital.

A short time later, they were both pronounced dead.

It is not known how four-year-old London - who was still missing until authorities located her body around 1.5km from where she entered the water the next day - fell in to the river.

According to KUTV, other children were "guided away from the fast, icy river rapids" but that DeDios and her daughter sat close to the edge under a small pedestrian bridge.

Utah County Sheriff’s Sgt. Spencer Cannon admitted the chances of finding the little girl were "bleak" due to the rough conditions of the river. The water is also extremely cold due to snowmelt coming off the mountains nearby.

"Just looking at the river here, it scares the daylights out of me just thinking of being in it," he told The Salt Lake Tribune.

The rescue effort was suspended overnight, and resumed first thing Tuesday morning. London's body was found around 11am.

Officials said they found the girl's body below the water level stuck under logs, Fox 13 reported.

Cannon added that parents need to watch their children at all times near water.

"Parents need to be so, so vigilant in situations like this," he said.

"Hang on to your kids for dear life."

A GoFundMe page has been set up by Brenda and London's family to help pay for funeral costs.

"Our angels...Brenda and London will forever be missed," the page said.

"Our family experienced a tragedy this afternoon at Bridal Veil Falls. Our mother, sister, wife and daughter lost her life today along with her precious baby London.

"The two were swept into the Provo river, and unable to make it out together and well."

Friends and family have also paid tribute to Good Samaritan Sean, who also lost his life trying to help the young girl.

"He would jump in for somebody else...He just had such a big heart," Sean's friend of ten years, Colton Swapp, told KUTV.

"He was amazing."