Stretching for 21,000 square miles, the Selous Game Reserve in south of Tanzania is one of the biggest wild areas left in the world. It is home to 15,000 elephants, and is a world heritage site.
But Tanzania urgently needs electricity and its president says nothing will change his plan to dam the river and more than double the country's power supply.
"We are not going to listen to the issue of environmental impact. The Selous Game Reserve is in Tanzania, it belongs to Tanzanians. We make these decisions for ourselves. The project will only take 3 percent of the area, and it will release 2,100 megawatts of electricity," said Tanzanian President John Magufuli.
But the park was already threatened by poachers, who have killed 90 percent of its elephants in 40 years. Conservationists say the dam could also cause serious and irreversible damage.
This week the World Heritage Committee asked the government to carry out full assessments of its impact and consider if there could be another way. But it is now unlikely the project will be stopped.
Tanzania needs electricity because there are still millions of people living without it. But the plant will put heavy industry in a protected area that the world recognizes as important.
The president says Tanzanians' needs must come first and it will go ahead, come rain or shine.