The Globo television network ran images of a first responder wading through knee-high water in Campo Grande carrying the little girl, Isabela Severo dos Santos, in his arms.
The spurt of water that resulted from the rupture reached a height of around 20 meters (65 feet), flooding the neighborhood with water up to 2 meters (6 ½ feet) high in places, the network said. Some 17 houses crumbled because of the flooding, and more than 140 people were forced out of their homes.
The cause of the break was not immediately clear.
During a visit by Rio Gov. Sergio Cabral, residents brandished signs berating the beleaguered official.
Cabral, who has seen his popularity plummet since mass nationwide protests swept Brazil in June, visited a local school where those displaced by the floodwaters had been taken. Speaking to gathered TV cameras, he pledged to give all possible assistance to those affected by the flood.
The incident came a day after Cabral appealed to critics to call off the protests they've been staging in front of his beachfront Rio apartment on and off since June.
Cabral said the demonstrations were disrupting the lives of his children, ages 6 and 11, and suggested the protests would be more appropriate in front of the governor's palace, his place of work.
"I'm making an appeal from the heart, as a father," Cabral said at a news conference Monday.
The demonstrators allege Cabral is corrupt and are calling for his impeachment, as well as an investigation into spending on the 2014 World Cup and 2016 Olympics, which Rio is hosting.
A poll by the Datafolha agency said Cabral's popularity has plummeted by 30 percentage points in the space of a few weeks. The poll showing his popularity fell from 55 points to 25 percent, was conducted on June 27 and 28 among 605 respondents and had a margin of error of plus or minus four points.
Cabral belongs to the centrist PMDB party, which is allied with the Worker's Party of President Dilma Rousseff, whose popularity has also taken a beating since the start of the protests.
A 10-cent hike in bus and subway fares in Sao Paulo initially triggered the wave of mass demonstrations, which rapidly snowballed into nationwide movement over a simmering public dissatisfaction about a disparate set of issues including government corruption, high taxes and poor government services.