The upper course belongs to the sertão, the semi-arid backland where rain is a rumour for much of the year. Here the Paraguaçu is less a highway than a lifeline, sustaining cattle ranches on ground that would otherwise support very little. For generations the river marked the transit point for herds moving out of the deeper sertão of Bahia and Minas Gerais toward the coast. The men who drove them, the tropeiros, built shelters along the route where animals and drivers could rest. Farmers built something else: small chapels, each dedicated to a patron saint, spaced along the way like punctuation. The pattern is still legible on the land - a whitewashed chapel where the track meets the water, and nothing else for kilometres.

Below the falls the river enters the Recôncavo, the crescent of fertile lowland wrapping the Bay of All Saints, and the character of everything changes. Sugar cane and tobacco grew here in quantities that made Bahia the economic centre of Portuguese America, worked by the largest enslaved African population in the early colony. Those people cut the cane, fed the mills, cured the tobacco and loaded the boats; the wealth their labour produced is what paid for the churches and convents that still line the riverbanks. Slavery in Brazil was not abolished until 1888, later than anywhere else in the Americas, and the Recôncavo's plantation economy was among the last to give it up. Walk through Cachoeira, São Félix or Jaguaripe today and the colonial architecture is genuinely beautiful. It was also built on that.

Heavy rain fell across Bahia in December 1989, and the Paraguaçu did what rivers do when the ground can hold no more. The flooding reached 175 towns and cities. Thirty-five people were killed. Two hundred thousand were displaced from their homes - a number large enough to lose its meaning unless you translate it back into households, into families who watched a familiar brown river climb their walls and kept climbing. For a waterway that gave the region its farms, its transport and its name, it was a reminder that the relationship runs both directions.

At the mouth, the Paraguaçu widens into the long narrow inlet of Iguape Bay before joining the Bay of All Saints. On 11 August 2000 the federal government created the Baía do Iguape Marine Extractive Reserve here - roughly 10,074 hectares spanning the municipalities of Cachoeira and Maragogipe, with boundaries redefined by law in October 2009. It is a category VI protected area, which means it exists not to exclude people but to secure the livelihoods of the ones already there. By 2002 some 900 families, about 4,960 people, depended on it. The division of labour is traditional: men fish, women and children gather shellfish from the mangrove mud at low tide. Shrimp, fish, oysters and mussels are harvested, sorted, sometimes smoked, and sold. It is one of the few conservation models in Brazil that treats a working community as part of the ecosystem rather than a threat to it.

Follow the Paraguaçu from source to sea and you get a cross-section of Bahia itself. Diamond country at the top, in the mountains of the Chapada Diamantina. Then the long dry middle where cattle and chapels are the visible economy. Then the sudden green of the Recôncavo, where the plantations were, where the colonial towns are, where the money and the suffering both concentrated. Then mangrove, then bay, then ocean. The Tupi called it the great river before any of that happened, and the name has outlasted the mills, the diamond rush, the tropeiro trails and the empire. Whatever else changes along its banks, the water keeps running the same direction.