The Bellevue Police Department had not released the identity the 60-year-old woman who died, the Dubuque Telegraph Herald reported.
Those injured included 13 children, the newspaper said. Ten of the victims were in serious or critical condition.
The Des Moines Register reported the incident happened near the end of the parade when the horses got spooked and galloped down Highway 52, staying close to the curb where parade-watchers were sitting. By the time Holly Specht of Peosta saw the horses they were free of the buggy.
"There were two horses coming at us at full speed, right at the curb," she told the Register. "People were grabbing kids. You looked up and it was just like a flash. Everybody was kind of in shock and panic."
The Telegraph Herald reported the buggy came unhitched when it hit a large street sign about eight blocks into the episode, tossing off two people on board, and the horses ran another two blocks before they hit a trailer.