The slow boat to Belem

From Manaus to Santarém, via Alter do Chau and Fordlandia, to Belém, we're almost only traveling by water. Close to 2500km on the Amazonian waters, the slow boats are the cheap boats. Crowded with hammocks, families and the sporadic tourist, they are really the only way to experience the Amazon.
Boats leave Manaus for Santarém every day, but only one boat a week leaves Santarém for Belém, and that boat starts in Manaus. As for our previous journey, it's possible to board the night before departure. Showers are available on board, so it's not too bad, but that would mean three nights of hammocks in a row, on one of the three very overcrowded decks. 
We chose to spend the night in Santarém instead, claiming a spot for our hammocks the night before.

A strange scene evolved once the boat hit the Amazonian delta. We veered off the river's main arm, into a narrow stream, more quickly reach Belém. Immediately, we saw the formerly mostly empty shores be noticeably populated, one or sometimes two houses next to each other every few hundred meters. Here, from each, one or two little boats, some with an engine, some with none, were paddling out to meet us. All little boats had kids on them, some not even adults.
Then, someone on our boat started throwing out tied shut plastic bags, which the river dwellers went after to pick up. This went on for hours.
Later, Natalia talked to the couple responsible for throwing out the packages (who, when they boarded together with us, were trying to sell religious crossword puzzles to the other passengers). It was their first time doing this, but they, they said, were only carrying on a tradition.

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