RIO DE JANEIRO (Reuters) - A severe drought has pushed river levels in Brazil’s Amazon region to record lows, leaving isolated communities dependent on emergency aid and thousands of boats stranded on parched riverbeds.

The drought fits a pattern of more extreme weather in the world’s largest rain forest in recent years and is, scientists say, an expected result of global warming. Last year, the region was hit by widespread flooding and in 2005 it endured a devastating drought.

The level of the dark Rio Negro, a tributary to the Amazonas river and itself the world’s largest black-water river, fell to 13.63 meters (45 feet) on Sunday, its lowest since records began in 1902, according to the Brazilian Geological Service. Only last year it hit a record high of 29.77 meters (98 feet).

The shallow water has exposed sandbanks and rocks and has made part of the river unnavigable. Life in the vast Amazon river network depends to a large extent on boat transport.

“People are lacking food because fish are dying in the warm waters. Nearly all boats are grounded -- only the smallest ones can navigate the waters,” said Rosival Dias, a coordinator with the Amazonas Sustainable Foundation environmental group who has visited affected areas.

“I’ve worked in the region about 30 years and never seen anything like the last few years. This has everything to do with climate change.”

(For a graphic showing rainfall patterns in Brazil this year, click on r.reuters.com/vus79p)

Amazonas state says the emergency has affected 62,000 people in 38 municipal areas, and that 600 tonnes of food aid has been distributed by plane and boat. The Brazilian government announced last week it was releasing 23 million reais ($13.5 million) in emergency aid.

Soy producers that rely on the Madeira river in Amazonas state to ship barges of the food product have been forced to divert loads at great expense to ports in the southeast of the country some 2,000 km (1,250 miles) away.

SEASONS “BREAKING DOWN”

Officials have voiced concern that the drought could affect turnout in Sunday’s presidential runoff election and are laying on extra boats able to navigate shallow waters to ferry voters to the polls. In some drought-hit areas, only about 50 percent of people voted in the first-round election on October 3.

“The drought is giving voters a lot of difficulty. Where normally they would have to take a two-hour boat ride to the nearest town, now they have to walk,” Pedro Batista, the director of the electoral authority in Amazonas state, was quoted as saying in the Correio Braziliense newspaper.

Some scientists say that this year’s drought may have been exacerbated by the El Nino weather phenomenon in 2009/10 and an active hurricane season in the Atlantic Ocean that may have sucked up moisture from the south.

But Daniel Nepstad, an U.S. ecologist with Brazil’s Amazon Environmental Research Institute, said the link between those phenomena and this year’s drought appeared less clear than it had been in 2005.

“I think it’s reason for some pretty deep concern over the Amazon ecosystem,” he said. “We’re seeing the reliability of the seasons in the Amazon break down.”

Deforestation of the Amazon has been falling steadily, and Brazil says it expects to announce a record low rate of deforestation for 2009/10.

Severe droughts, which kill trees and make them more vulnerable to fires, are part of a damaging cycle that some scientists believe could bring the Amazon closer to a “tipping point” at which its destruction becomes self-sustaining.

Destruction of the forest fuels global warming because trees release their carbon when they die. The 2005 drought released more greenhouse gas than the annual emissions of Europe and Japan, an international study found last year.