So flat is the landscape of Amazonia that at Tabatinga on Brazil’s western frontier with Colombia, the Amazon River is only 73 metres (240 feet) above sea-level – and it still has 4,660 kilometres (2,900 miles) to go.

[ From the Amazonia exhibition featuring the photographs of Sebastião Salgado at the Science Museum London, November 2021]

That makes the angle of drop 0°31’43” – about one half of a degree.