Few cities boast of natural diversity as rich as Mumbai's: we have the jungles of the Sanjay Gandhi National Park (SGNP), hills, rivers, creeks, mangrove forests, and the sea. It is also the world's only city with a leopard habitat in its centre, not to mention 162 species of birds and butterfly species.

Part of this rich heritage: the city's four major rivers, Dahisar, Mithi, Oshiwara, and Poisar. The Dahisar and the Poisar originate from the Tulsi lake, the Oshiwara begins its journey in Goregaon, and the Mithi in the Vihar and Powai lakes.

None of them reach the sea unsullied; slums, industrial areas, cow sheds all dump untreated, often toxic, waste into them. Instead of channels for excess rainwater, they are now stinking, garbage-lined, slime filled sewers. Aside from the assault on the senses, they are also reservoirs of disease: residents along their banks are no strangers to malaria, dengue and various skin and respiratory ailments. Generations of Mumbaikars don't even know them as rivers; all they see is smelly 'nullahs' that flood in the monsoon. In fact, says Gopal Jhaveri, Borivali resident and co-founder of a movement called River March, the Chitale Commission — set up to recommend remedies following the 2005 floods — and said the Brihanmumbai Municipal Corporation, in its development plans purposely refers to river systems as nullahs so river laws don't affect them. But, he says, "The Commission clearly says a 'nullah' is a river tributary, derived from an Urdu word, and does not mean a sewer line."

"In any village or town in India, people use the river water for drinking, or enter the river for spiritual purposes," says Avinash Kubal, Director, Mahim Nature Park. "But there is no such reason in Mumbai, so we are actually disconnected from our rivers."

On World Environment Day, The Hindu's photographers trace the journey of the four rivers — and a fifth in the metropolitan region, the Waldhuni in Ulhasnagar — to see just where the rot is.