Emergency teams hope that it will give them more time to deal with the disaster.
On Sunday, four million litres of crude oil escaped from a burst pipeline into the river making it the biggest oil spill in 25 years.
A Petrobras statement said the spillage was brought under control by installing a number of floating barriers and other equipment at eight key points on the river.
Oil is still seeping through the floating barriers, but the slick has been moving along the river slower than expected.
Emergency teams hope that it will give them more time to deal with the disaster.
The crude oil has been flowing down the southern Iguacu River, endangering drinking water, farm land and animal life along a 230 km (140 mile) stretch.
Dead fish, birds and mammals coated in oil are washing up on the river banks, according to environmental officials.
State-owned oil company Petrobras is to be fined $28m for the spill - the maximum allowed for environmental disasters.
Regional threat
Brazil's Foreign Ministry said it had spoken to the governments of Argentina, Paraguay and Uruguay about its efforts to prevent the oil spreading over Brazil's borders.
The leak happened 20km upstream from the state capital of Curitiba, so its 1.5 million inhabitants are not in any immediate danger.
But authorities warned that by Wednesday the slick could reach Uniao da Vitoria, a town of 75,000 people, about 200 km away.
The city depends on the Iguacu for drinking water and has an important hydroelectric dam.
Environment agency president Jose Antonio Andreguetto said: "Plants and animals are significantly affected.
"The rehabilitation of the ecosystem will take a long time and will be very difficult."
'Absolute negligence'
Petrobras President Henri Phillippe Reichstul said the company accepted full responsibility for the accident and that "unlimited" resources would be used to repair the environmental damage.
But the company may face a further heavy fine.
"We cannot allow a company the size of Petrobras to get away with at least two serious accidents in less than a year," Environment Minister Jose Sarney Filho said. "This is absolute negligence."
Petrobras has been aggressively expanding its activities to keep a competitive edge since the government ended its 46-year monopoly on oil exploration last year.
Environmentalists say the company's pipelines are poorly maintained, while unions representing workers at the company say a recent 50% reduction in staff could have a lot to do with the recent leak.