By Paulo Cabral
BBC Brazilian Service
Explorer Captain Richard Burton, wrote in August 1867, in Sabara, Brazil: "Here a man can catch a half a dozen sprat like Piabas simply by heaving up a bucketful of water and throwing it upon the river bank."
With much pollution and little water in Brazil's Velhas river, the "flourishing fishing industry" envisaged by Richard Burton remains an unfulfilled dream for the communities along its banks.
A few fish are still there, but they are tainted by industrial effluent and human sewage, just like the Velhas river that Burton navigated for 600 kilometres to reach the Sao Francisco river.
There, river fish face environmental change of a different kind, not from water pollution but the lack of seasonal floods, which local fish require for spawning.
"The many dams along the river prevent the big floods that once allowed so many fish to reach lagoons on the banks and reproduce in a safe environment," says Alex Godinho, a fish biologist from Federal University of Minas Gerais.
"Before, a fisherman could catch at the very least five kilos, but up to 50 kilos, in just one go. Nowadays they are lucky if they get three kilos in a full day of work."
There are seven hydroelectric plants, with their huge dams, along the 2,700-km course of the Sao Francisco.
These dams have displaced people and had a significant impact on the environment, but today could also be used to save the fish.
"We could create artificial floods using the water from the reservoirs," Mr Godinho suggests.
The Sao Francisco flows along the canyon in northeastern Brazil
One problem, though, is that all this water is currently used for electricity production.
"Today water is an asset and it would have to be somehow bought from the power plants to be used in an artificial flood," says Carlos Alves, a biologist from Manuelzao Project - an NGO focused on social and environmental issues on the Sao Francisco basin.
Studies are needed to assess both the cost and the benefits of such a project.
"Hopefully, we will be able to prove that environmental, economical and social gains can compensate for the amount of water and money that would have to be spent on it."
But Mr Alves believes that the project is possible.
"A similar initiative was successfully tested in the United States, on the Mississippi river," he adds.
The decrease in the flow of water is being felt all the way up to the mouth of the river.
"Before that the flow of fresh water was so strong that it could be detected up to seven kilometres out to sea. Now it is the ocean water that has encroached upon the river, almost a kilometre inland," says a tourist guide in the small seaside town of Piacabucu.
Biologist Carlos Alves says that flooding would help to reestablish the fish population
The beaches there are pristine and the vegetation virtually untouched, making it a great destination for eco-tourism. But to eyes more used to that landscape the changes caused by human activity are clear.
While we sailed to the mouth of the river, John Lennon - son of Beatles fans, as you would imagine - pointed to a lighthouse surrounded by water.
"Can you see that lighthouse? Some years ago it was 500 metres on shore and now it is partly under water."
The Brazilian Government is also considering taking water from another river in the Amazon basin and using it to replenish the Sao Francisco, through gigantic water channels.
This would increase water flow and even allow the diversion of a part of the Sao Francisco river to other dry areas of the country.
The social benefits, in providing much needed irrigation for agriculture, would be undeniable. But there are serious concerns about how political interests might distort the project, and its potential impact on the environment.
"Transferring fish from one basin to the other, as would happen with this channel, could be disastrous for the river's sensitive ecosystem," says Carlos Alves.
Sewage from four million people is thrown into the Velhas river
To clean up the Velhas river authorities will have to tackle another complicated issue: the limited treatment of sewage in the region.
When Burton visited the Velhas river, he wrote that just a bucket filled with water there would bring at least half a dozen fish.
"Today the water is full of plastic bags, plastic bottles, shoes and a lot of pollution that we cannot see, but smell," says Mr Alves.
The biologist explains that nowadays there are two sewage treatment plants that deal with the output of about one million people in Belo Horizonte - quite a lot but far less than is required for the region's four million inhabitants.
But contaminated or not, poverty forces the riverbank community to live off fish that swim in those waters, which, according to a local, is only edible "dressed with a lot of lemon to disinfect the flesh and remove the smell".
Paulo Cabral travelled to northeastern Brazil for the BBC, keeping an online journal along the way. His three-part series, Blogging in Brazil, starts on BBC World Service, on 6 August.