BBC reporter Paulo Cabral is travelling along Brazil's Sao Francisco river, following in the footsteps of Victorian explorer Sir Richard Burton. Each week day Paulo will be posting a diary entry on the web, and responding to a selection of your e-mails.
3 July: Paulo Afonso
In Paulo Afonso, it's not only the electricity provided by hydro-electric power plants that can be switched on and off. At the touch of a button, so can the Paulo Afonso waterfall - Richard Burton's Brazilian Niagara.
Many waterfalls in Brazil have been submerged by artificial lakes for energy production, but what's happened at the Paulo Afonso Falls is completely the opposite. The falls remain - it's just the water that's gone.
In 1867 Burton wrote that the falls "fill with what seems as not water, but froth of milk, a dashing and dazzling a whirling and churning surface-less mass".
The Paulo Afonso falls with the water switched off...
Now there is just a huge and dry rock. It takes quite a leap of the imagination to see it as it once was. But the Paulo Afonso dam's floodgates can be opened, and then the falls return to life.
In general the local population does not regret the loss of the falls as they have traded them in for an energy supply covering a vast area of north eastern Brazil, and for jobs in a more limited region.
"My father worked to build this dam, and afterwards on the operation of the plant. I work here now and I hope my son will have a job here in the future," said one of the plant's supervisors.
Talking to people in town, I felt their hearts equally divided between the river's natural beauty and the impressive engineering works that has brought the plant to life. In a CD recorded by employees of the company, songs praise both.
... and with the water switched on
I did not manage to visit the vast Sobradinho Dam, higher up the Sao Franscisco River, as suggested in an e-mail by Jose Carlos Andrade in Brazil.
He recalls the curious sensation he experienced there, separated by just one steel plate from the waters of its enormous artificial lake.
But I can assure Jose Carlos that wandering in the tunnels of the Paulo Afonso plant - 82 metres under ground and water - was quite an impressive experience as well!
In Paulo Alfonso, people hope that an embryonic government project to increase the flow of water in the Sao Francisco River will take place.
That would allow the amazing natural force of a big waterfall to live side by side with Paulo Alfonso's man-made electricity.
Or, as Richard Burton imagined: "the realised idea of power, of power tremendous, inexorable, irresistible. A spellbinding contrast of water in madding haste to escape, with the frail bits of rainbow hovering above."
2 July: Cabrobo
In the book of his journey along the Sao Francisco River Valley, Captain Richard Burton noticed the potential of this region for hemp plantations.
Burton was right, only nowadays a big part of this area is known to modern Brazilians as the "Marijuana Polygon".
The vast wealth Burton imagined from hemp production in the 19th century in fabrics and rope-making never fully materialised.
And instead, the hemp grown here by poor peasants boosts the trade of drug dealers in the big Brazilian towns further south and east.
In the wilderness of the Caatinga it's easy to hide marijuana crops
"Most of these growers are not professional criminals, but poor farmers that have no money to invest in legal crops", said Captain Jose Mario, the chief of police in the town Cabrobo.
"So they turn to the drug barons for money to produce Marijuana and generate cash to provide for their families".
The police can tackle arrest the farmers and destroy their plantations, but it's unfair distribution of wealth in Brazil that creates the problem.
Solving it will take more than mere police work.
Because of the drug trade, violent crime is rife here, and the main road through the Marijuana Polygon is considered one of the most dangerous in the country.
Until recently, muggings were common along its potholed route, where slow drivers fearing for their tyres would fall easy prey to gun toting highway robbers.
It was a tense drive through for me, with one eye on the road and the other scanning the roadside dodgy characters.
Chief of police Jose Mario provided welcome protection
I was relieved when, after reaching Cabrobo, its police chief offered the services of one of his men to escort me the remaining 150km out of the polygon.
"Sometimes bandits block the roads to force cars to stop, or simply jump from the roadside with heavy weaponry," said my escort, soldier Romero. I couldn't help wondering what my newfound security guard would be able to do if something like that actually happened.
Thinking back to the town of Cabrobro, however, it was hard to believe I was in a major centre of drug production and in any kind of danger at all. It's clean, calm and friendly.
Instead of drug dealers on the streets, I noticed an unusual number of joggers passing by along the town's flowered main street.
Send your questions or comments to Paulo using the form below. He will answer a selection in his daily blog.
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