One of the most important tributaries of the Amazon river has fallen to its lowest level in over a century, following a fierce drought that has isolated tens of thousands of rainforest inhabitants and raised concerns about the possible impact of climate change on the region.

The drought currently affecting swaths of north and west Amazonia has been described as the one of the worst in the last 40 years, with the Rio Negro, which flows into the world-famous Rio Amazonas, reportedly hitting its lowest levels since records began in 1902 on October 24.

In 24 hours the level of the Rio Negro near Manaus in Brazil dropped 6cm to 13.63 metres, a historic low.

The Solimoes and Amazonas rivers have also seen their levels plunge since early August, stranding village dwellers who rely on the Amazon's waterways for transport and food and marooning wooden boats on brown sand banks.

According to local authorities nearly half of Amazonas state's 62 municipalities have declared states of emergency, among them Manaquiri, one of the worst hit areas during the last major drought in the region in 2005. That year thousands of families were forced to abandon their homes and schools closed for lack of students.

Authorities say around 62,000 families have been affected by this year's receding rivers and on October 22 the federal government announced $13.5m in aid for the region.

More intense near border

The problem has been particularly intense up river from state capital Manaus towards the border with Peru and Colombia. But the area around the city has also been badly hit. In Iranduba, 24km from Manaus, authorities are reportedly planning to hack a small road through the rainforest in order to reconnect their community with the outside world.

“In my whole life I have never seen a drought like this one,” 50-year-old river-dweller Manoel Alves Pereira told the local A Critica newspaper.

Colonel Leite, from Manaus' Regional Air Force Command, said two Hercules cargo planes had ferried around 830 tonnes of food aid to isolated regions near the Amazon towns of Tefe and Tabatinga.

“Medium and large boats have not been able to reach various places across the Amazon,” he said. “Our planes take the supplies to the airport and from there the transport has to be done in small boats or on foot — these are the only ways of reaching some communities.” Meteorologists and activists are divided on the drought's causes — some point to hurricanes in the Atlantic which may have sapped humidity from the Amazon while others blame forest fires for stifling rainfall or speculate that early effects of global warming may already be reshaping the region's climate patterns.

Rafael Cruz, a Greenpeace activist in Manaus who has been monitoring the drought, said while the rise and fall of the Amazon's rivers was a normal process, recent years had seen both extreme droughts and flooding become worryingly frequent.

Although it was too early to link the droughts to global warming directly, Cruz said such events were an alert about what could happen if action was not taken.

“The photographs we are seeing of boats stranded in dry riverbeds are photographs that show the face of climate change, that show the impact climate change could have on the 20 million people who live in the Amazon region.” “If this situation continues the state of Amazonas will live in a permanent state of emergency. The changes in people's lives would be horrific.” — © Guardian Newspapers Limited, 2010

Meteorologists and activists are divided on causes of the drought, with some pointing to climate change as a factor.