Hundreds of indigenous people demonstrated in Brazil’s capital yesterday, handing the president half a million signatures on a petition against the proposed Belo Monte dam in the eastern Amazon. Brazil’s government says projects like this are needed to spur economic growth -- but building the world’s third-biggest hydroelectric dam in the heart of the world’s first-biggest rainforest has sparked criticism. Environmentalists, indigenous advocates, even Hollywood types like “Avatar” director James Cameron have spoken out. Critics say the project will displace indigenous people, swamp too much jungle and, as we wrote recently, the thing may not even work. Final approval on Belo Monte is still pending, but last month the government gave a partial go-ahead to begin clearing trees on the site.