Dozens of law enforcement officers in Evanston were searching for the girls, Lyric Cook-Morrissey, 10, and Elizabeth Collins, 8, the Des Moines Register reported.
Family members told the media the scents of the two girls had been traced by canines at Meyers Lake in Evansdale.
Meanwhile, crews were draining the lake, which is 21 feet deep in places, in the search for the girls.
An Evansdale resident reported seeing the girls riding their bikes between noon and 3 p.m. Friday, and an Evansdale firefighter found the girls' bikes and Elizabeth's purse and cellphone about 4 p.m. on a recreational trail near the southeast corner of Meyers Lake, the Register said.
Rick Abben, a chief deputy with the Black Hawk County Sheriff's Department, would not elaborate on any clues found with the bikes, saying only they were processed. While the bikes were found on the recreational trail, he said, "that doesn't tell me [the girls] have been there, just their bikes."
Abben said authorities had "no reason to believe there's been any foul play at this time."