 The Sao Francisco river also provides hydroelectric power |
Luiz Flavio Cappio, 59, is protesting against plans to divert some of the water from the Sao Francisco river.
The diverted water will be pumped into four arid states in the extreme north-east of Brazil.
The bishop has said that he is putting his life in the hands of President Luiz Inacio Lula da Silva.
Bishop Cappio has been refusing food since Monday lunchtime.
He is drinking small amounts of water but has made it clear he is prepared to die if his demands aren't met.
A written declaration even names the exact spot where his body is to be buried.
Billion dollar plan
His protest is against one of Brazil's most ambitious but controversial environmental projects.
It would see water from the 3,000-km (1,800-mile) Sao Francisco river diverted via a series of canals and aqueducts to four drought-prone states in north-eastern Brazil.
The project will cost more than $2bn.
Opponents say the scheme will benefit only the wealthiest landowners in the north-east and reduce the capacity of dams on the Sao Francisco river to generate hydro-electric energy.
Bishop Cappio has called on President Lula to halt the project before final approval is granted by the country's environment agency.
By coincidence, President Lula was visiting the bishop's home state of Bahia on Wednesday.
He has made no public comment but it is understood the presidential palace is following the case closely.
