MR. DOS SANTOS, soft-spoken and bespectacled, insists that he has never seen any pirates navigating the Tietê or its tributaries. But he has glimpsed other living beings. Herons tiptoe along some riverbanks. He said that capybaras, the world’s largest rodents, roll in the mud along some stretches of the Tietê and Pinheiros. Alligators have been known to emerge from the rivers, weary but resilient.

One of the most astonishing sights of all, Mr. dos Santos said, was a man in São Miguel Paulista, a gritty district on São Paulo’s eastern fringe, who went by the name Pezão and dived into the Tietê without any gear in search of metal to sell to recyclers. “If there’s anyone who deserves recognition, it’s that guy, not me,” Mr. dos Santos said.

Still, he said he held out hope that the stubborn presence of life along São Paulo’s rivers might reflect the latest phase in their existence: the attempts to resurrect them. Since 1992, the authorities have been advancing with a painstakingly slow project to clean up the Tietê and Pinheiros.

Political leaders here contend that the cleanup effort, financed with loans from the Inter-American Development Bank, is going swimmingly. Gov. Geraldo Alckmin even said this year that by 2015, boats could start taking tourists down the Tietê for glimpses of São Paulo’s wonders. (“The problem is removing the smell,” he acknowledged.)

Brazilian scientists point to precedents of restoring vital waterways, as Paris has done with the Seine or London with the Thames, allowing salmon to thrive there decades after they had disappeared.

Cleaning the Tietê and its tributaries, however, offers complications that are in a league of their own, and paramount among them is access to sewage treatment. This deficiency plagues Brazil’s only truly global city, in which hedge funds inhabit hulking postmodern skyscrapers, well-heeled consumers stream into luxury shopping malls and immigrants are as likely to speak Castilian Spanish as Quechua.