CHICAGO — For weeks, residents of Davenport, Iowa, looked warily toward the flooded Mississippi River as it encroached on their downtown, the water kept at bay only by temporary barriers lining the street.

On Tuesday, those barriers broke, sending murky river water rushing into businesses and forcing residents to scramble to safety. About 30 people who did not make it out in time were rescued by firefighters.

“The whole city had a very frantic feel about it,” said Rebecca Nicke, a co-owner of Abernathy’s, a vintage store located about half a block from the barrier, who said she ran for her vehicle and drove toward a hill when she heard the water coming.