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The government is giving lumber seized from illegal Amazon loggers to the ribeirinhos so they can build their shacks higher.

The lumber also is going to slum dwellers whose stilt shanties litter big jungle cities such as Manaus — an industrial metropolis of 1.7 million that’s also a jumping-off point for jungle tourism — so they can try to stay dry as the mighty Rio Negro, an Amazon River tributary, approaches a record high-water mark set in 1953.

The floods are driving anacondas and scorpions to higher ground and closer to humanity as they search for food. While many of the animals in the Amazon thrive on the higher water, experts warn that more droughts could sharply reduce the range of species such as pink Amazon River dolphins, already under pressure because of deforestation and pollution.

“We could be looking at an Amazon that is much more populated by animals that are generalists and can move through human landscapes, and some of the more sensitive species will be caught in islands of habitats,” Mr. Nepstad said.

In Trizidela do Vale, floodwaters reached the red tile rooftops of many homes and left more than half of the town’s 20,000 residents homeless. Many sheltered in cow pens used for the town’s annual cattle fair until officials closed them as unsanitary.

In Manaquiri, ribeirinhos whose crops were destroyed paddle into the town of 19,000, seeking government handouts of food, medicine and clothing.

“We are used to floods and droughts and know how to coexist with them, but we are not used to them happening so swiftly and lasting so long and causing so much damage,” said schoolteacher Gleicimeire Freire, who distributes aid with the Roman Catholic Church. “This is what is scaring us.”

In southern Rio Grande do Sul state, bordering Argentina and Uruguay, many farmers say the driest weather in 80 years has withered their corn and alfalfa. Winter grass for cattle couldn’t be planted, and milk production has suffered, said Darcisio Perondo, a congressman who represents the state.

“In some villages there wasn’t enough water for people to drink, and in some towns they had to get water from the large rivers and tote it by truck for the cattle,” Mr. Perondo said.

He called the situation a calamity, but isn’t sure whether global warming is to blame.

“Anyone who reads the Bible knows that floods and droughts are cyclical,” he said. “I just don’t know if global warming is causing this.”