The disaster happened in the town of Hildale on the border with Arizona, the Utah Division of Emergency Management said on Twitter.
A big wall of water hit people in their cars and washed them downstream, Hildale Fire Department Assistant Chief Kevin Barlow said, according to ABC News.
The network said another six people were reported missing.
Crews searched for missing people in the flood waters but scaled back the operation at nightfall because conditions were still dangerous.
The rush of water left streets clogged with mud, rocks and debris, ABC said.
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