An acute drought in Brazil has hit the famed horseshoe-shaped Igauzu falls which straddle two countries, cutting back the tumbling waters to reveal the rocky sides.

Only a third of the usual volume of water is now flowing over the top of the stunning falls, which were listed as a world heritage site by UNESCO in 1984 and border both Brazil and Argentina, Globo television said.

At the foot of the falls on the Brazilian side, the bottom of the Parana river is now clearly visible, allowing environmentalists a rare chance to clean up mountains of accumulated trash.

The falls, which are actually made up of 275 waterfalls stretching some 2.5 kilometers (1.5 miles), are taller than the Niagara Falls and twice as wide.

They provide a panoramic backdrop to the tropical rainforest region, with an average of 553 cubic feet of water per second from the Iguazu River thundering some 269 feet over the falls and then draining into the Parana.

Divers have been cleaning up the garbage which has collected in the Parana, finding everything from cameras to combs, CDs and batteries as well as plastic bottles, tin cans and umbrellas.

Some of the trash has floated down river from other towns, but most has been dropped by tourists, said environmentalist Tassio Lima.

"Under the algae, we have found lots of coins. It's all rubbish and the tourists shouldn't be throwing them into the river," he added.

Southern Brazil has been hit by its worst drought in 80 years, leading authorities to declare a state of emergency in some areas, while the usually arid northeast of the country is suffering from floods.