The apex environment watchdog also said that unless the excessive extraction of water and discharge of high pollutants are controlled, it would be difficult to restore Ganga to its original pristine condition.
The E-flow defines the quantity, timing and quality of water flow required to sustain freshwater and the estuarine ecosystems, besides human livelihood.
A bench headed by NGT Chairperson Justice Swatanter Kumar directed the Environment Ministry, Ministry of Water Resources and the Central Pollution Control Board to provide complete information on e-flow to be maintained in the river.
"Ministry of Environment and Forests, Ministry of Water Resources and CPCB will take a clear stand as to what should be the minimum environmental flow of river Ganga in Segment-B (Haridwar and Unnao)," the bench said.
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The NGT fixed the matter for hearing on April 19.
Earlier, the Yogi Adityanath-led Uttar Pradesh government favoured the shifting of British-era tanneries releasing toxic wastes into the river Ganga at Kanpur.
Last year, the then Akhilesh Yadav government had opposed the idea of shifting of over 400 tanneries giving employment to over two million people, saying the moving the tannery hubs to some other place was almost "next to impossible" due to paucity of land.
In a detailed report covering various aspects of contamination in the river, CPCB had informed the NGT that the Ganga, spanning a distance of 543 km between Haridwar and Kanpur, was affected by 1,072 seriously polluting industries which were releasing heavy metals and pesticides.
The green panel has divided the work of cleaning the river in different segments - Gomukh to Haridwar (Phase-I), Haridwar to Unnao (termed as segment B of Phase-I), Unnao to border of Uttar Pradesh, border of Uttar Pradesh to border of Jharkhand and border of Jharkhand to Bay of Bengal.