Madison Leigh Wallace jumped into the Big Sioux River at Falls Park on Thursday night after her brother either fell into the frothing water or became obscured by the gathering foam, which reached as high as 10 feet, Sioux Falls Fire Chief Jim Sideras.
Her brother, Garrett, had been climbing on slippery quartzite rocks at the edge of the river at the park in Sioux Falls.
"It's hard to say if he actually ended up in the water or not," Police Chief Doug Barthel said.
After Madison jumped in, Sioux Falls resident Lyle Francis Eagletail, 28, went into the freezing water to help the two, Barthel said.
A friend of Eagletail's who witnessed the tragedy, 21-year-old Napoleon Ducheneaux, said his friend was holding onto the girl and boy by their hands before his hands began sliding. Then, he just "slipped and disappeared," Ducheneaux told The Associated Press late Thursday.
Eagletail's body had not yet been recovered as of Friday evening, when crews suspended their search because of dangerous river conditions. They planned to resume their search Saturday.
Madison was from Vermillion, about 60 miles south of Sioux Falls.
Witness accounts differ on whether someone pushed the boy out of the water or he popped up on a rock before being pulled ashore. Emergency workers carried the boy away from the river wrapped in a blanket and he was not injured, Sideras said.
"These people literally jumped in without thinking of their own safety and trying to rescue that child," Sideras said. "It's a very noble act that they did, and they probably contributed to saving that boy's life."
Nevertheless, the fire chief said the optimum way to help someone who falls into water is to stay on the shore and mark where that person went in.
Rescue crews have been working against the river's strong current, a thick sheet of ice that firefighters are trying to break into pieces, and large amounts of foam, which firefighters were blowing away with water hoses, Sideras said.
The water temperature hovered around freezing, putting emergency crews at risk of hypothermia.
The city of Sioux Falls is named after the Big Sioux River's cascading waterfalls in Falls Park, a tourist attraction where people often picnic or pose for wedding photos.
It's a popular spot in the summer and spring, drawing about 525,000 visitors annually, according the local visitors' bureau. For the first time in months, the temperature rose to around 50 degrees in Sioux Falls on Thursday.
The park was closed Friday as crews continued their recovery effort.
Fatal accidents are rare at Falls Park. In 2006, the body of 29-year-old Travis Hallan was found just north of the falls after his canoe tipped over. In 1999, 26-year-old Slavisa Andric drowned after losing his footing on rocks at the park. A bystander in 1997 pulled the body of 43-year-old Omar Iasi Ibrahim Warsame from the water below a bridge where he had been fishing.
Barthel said the city could perhaps prevent accidents by fencing off the entire area, but there's some inherent danger with a fast-moving river meandering through the city.
"The Falls is the crown jewel of our city," Barthel said. "I don't think we want to get to a position where we're going to be totally fencing it off."
The city rescheduled its annual lighting of The Falls green to mark St. Patrick's Day until Sunday because of the search effort.
Associated Press writer Kristi Eaton in Sioux Falls and Blake Nicholson in Bismarck, N.D., contributed to this report.
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