Swami Nigamananda Saraswati, a member of Matri Sadan, an ashram on the banks of the Ganga in Haridwar, had been on a fast for almost four months, died on Monday.
He was protesting against the illegal mining that is polluting the Ganga.
The Swami had recently gone into coma and was admitted to the Jolly Grant Hospital in Dehradun. Baba Ramdev had reportedly visited the Swami in hospital.
Earlier also, for almost three months in 2008, Swami Nigamanand Saraswati had gone on a fast to save Ganga from the illegal mining.