Caleb Linn, whose fourth birthday is next month, was among 11 children working on a cleanup project Saturday at a bridge in the Kia Kima Boy Scout Camp in Fulton County, about 120 miles northeast of Little Rock.
The children were not affiliated with the Boy Scouts but were staying at the main campsite about half a mile from the bridge. When five children left to return to their cabins before noon, Caleb asked if he could try to catch up with them, and he hasn't been seen since, state police said.
Dive teams were searching the Spring River for a second day on Monday, and police and volunteers were combing through the surrounding wilderness.
Sheriff Buck Foley said crews were clearing a log jam in the river Monday near where Caleb was last seen.
"He was on the concrete slab of the bridge at the last place they saw him," Foley said.
Authorities also set a net downstream that could catch the boy's body if he did fall into the river, but it's possible the body could have passed that point before the net was put up, said Major Todd Smith, the assistant chief of enforcement for the Arkansas Game and Fish Commission.
"It's like looking for a needle in the haystack," Smith said. "There's no way to search the whole river."
Smith said the river is clear but moving swiftly, with water temperatures in the lower 60s.