The Central Pollution Control Board’s latest assessment of 17 polluted rivers in West Bengal places Kolkata’s Adi Ganga within a wider river crisis shaped by sewage, shrinking flows, encroachment and uncontrolled urban pressure. The findings show that the problem is not limited to one degraded waterway or one failed clean-up project: across the state, several rivers are carrying pollution loads that threaten aquatic life and weaken the ecological systems on which towns and cities depend.
The rivers identified as having critically polluted stretches are the Hooghly, Churni, Damodar, Jalangi, Mathabhanga, Barakar, Dwarakeshwar, Dwarka, Rupnarayan, Teesta, Bidyadhari, Kansi, Silabati, Kalijani, Mayurakshi and Karola. Together, they represent a network that links Kolkata’s metropolitan landscape with industrial areas, agricultural districts, smaller towns and rapidly urbanising settlements.
Adi Ganga is the most visible urban example. The waterway runs through Kolkata and its surrounding areas, where the river and canal network receive large wastewater loads. A sample collected by the West Bengal Pollution Control Board at Jirat Bridge in November last year found the water blackish and foul-smelling. The sample recorded biochemical oxygen demand, or BOD, of 18.5 milligrams per litre, dissolved oxygen of 2 milligrams per litre and faecal coliform of 84 lakh MPN per 100 millilitres.
These measurements describe more than poor water quality. BOD indicates the amount of oxygen required to break down organic matter in water. When the organic load is high, microorganisms consume more oxygen. Low dissolved oxygen, meanwhile, reduces the capacity of a river to support aquatic life. In the Jirat Bridge sample, the combination of high BOD and low oxygen indicates a waterway under severe organic stress.
The wider CPCB assessment records even higher BOD levels in other stretches. The Churni, between Bijoypur and Ranaghat, recorded a maximum BOD of 20.8 milligrams per litre. The Vidyadhari stretch between Haroa and Malancha recorded 29.6 milligrams per litre, while the Mahananda near Siliguri recorded 31 milligrams per litre. The figures suggest that pollution is not confined to the dense urban core of Kolkata. It is distributed across river systems that pass through different settlement patterns and land uses.
The Hooghly has a polluted stretch extending from Berhampore to Haldia, a long corridor that includes major settlements and economically important areas. The river is central to Kolkata’s geography, industry, transport history and water environment. Pollution along such an extended stretch indicates that the challenge is regional rather than limited to isolated discharge points.
River scientist Kalyan Rudra said high BOD reflects a heavy load of organic pollutants, while low dissolved oxygen shows that a river is losing its ability to sustain aquatic life. He also linked the problem to the lean season, when natural flows fall and there is less water to dilute sewage entering the river. This seasonal dimension is important for urban management. A river may appear to have sufficient volume during periods of high rainfall, but its ecological condition can deteriorate sharply when flows reduce and wastewater remains constant.
The result, Rudra said, is that a river can gradually begin to function more like a wastewater channel than a living water system if untreated or inadequately treated sewage dominates its flow. That distinction exposes a limitation in how river restoration is often understood. Treating sewage is essential, but treatment infrastructure alone cannot restore a river if the waterway has lost flow, space and ecological connectivity.
The condition of Adi Ganga illustrates this institutional challenge. The river is part of an urban drainage landscape that receives wastewater while also facing the effects of population growth and urban expansion. Restoration projects have struggled to keep pace with those pressures, according to the report. The problem is therefore not simply whether a project exists, but whether the project’s scale, operation and long-term maintenance match the growth of the city around it.
Sewage management is divided across multiple administrative responsibilities, even when the environmental consequences appear in one river. Wastewater collection, treatment, drainage, land-use control, encroachment management and river restoration may involve different agencies and levels of government. A river can remain polluted even when one part of that chain improves, because untreated inflows, blocked channels, inadequate treatment capacity or illegal dumping continue elsewhere.
This is why the pollution figures raise a governance question as much as an engineering question. If the river is treated only as a drain, the response is likely to focus on intercepting wastewater and removing visible waste. If it is treated as an ecological system, the response must also account for flow, floodplain space, siltation, channel continuity and the relationship between the river and the surrounding urban landscape.
Experts cited in the report say Bengal’s rivers are losing ecological resilience because of encroachment, siltation, altered channels, reduced dry-season flows and shrinking floodplains. These pressures reinforce one another. Encroachment reduces the space available for water movement and flood absorption. Siltation can alter channel capacity. Reduced flows leave less water available to dilute pollutants. Altered channels can weaken the connection between the river and its floodplain.
The consequences are not limited to aquatic organisms. Urban residents experience degraded rivers through foul odour, visibly contaminated water, blocked or overloaded drainage channels and the loss of public environmental assets. Rivers and canals that once formed part of a city’s natural drainage and flood-management system can become liabilities when their carrying capacity is reduced and wastewater becomes their dominant function.
The numbers also show why a citywide approach is insufficient. Kolkata’s sewerage and canal systems are connected to a wider basin and river network. Pollution in the Hooghly, Bidyadhari or upstream stretches can affect the ecological condition of downstream areas. Conversely, local improvements may have limited impact if pollution continues to enter from connected settlements or if seasonal flows remain inadequate.
Naba Dutta, general secretary of Sabuj Mancha, said sewage treatment was necessary but could not by itself revive a river. He called for adequate ecological flow, floodplain space and protection from encroachment, waste dumping and indiscriminate construction along riverbanks. His formulation places river restoration within the broader built-environment debate: the question is not only how cities dispose of waste, but how they occupy land around natural systems.
That approach also changes the meaning of restoration. Cleaning a river’s surface or constructing treatment facilities may produce measurable short-term improvements, but ecological recovery depends on sustained control of pollution at source and protection of the river’s physical space. Without those conditions, a river may repeatedly return to a degraded state even after expensive interventions.
The CPCB figures do not, by themselves, establish the performance of every treatment plant or identify the precise contribution of each pollution source. They do establish that multiple river stretches are experiencing serious organic pollution, with particularly high BOD recorded in the Churni, Vidyadhari and Mahananda. The report also links the crisis to sewage discharge, low flows, urbanisation and the loss of ecological resilience.
The next policy challenge is therefore coordination. River revival requires pollution monitoring, sewage treatment, flow protection, floodplain management and enforcement against encroachment to operate as parts of one system. If these functions remain separate, cities may continue to invest in individual projects without repairing the wider ecological network.
West Bengal’s 17 polluted rivers show that the state’s water crisis is also an urban planning crisis. The evidence points to waterways being asked to absorb the costs of expanding settlements while losing the flow and space needed to remain ecologically functional. The future of Adi Ganga, the Hooghly and the other identified rivers will depend on whether restoration is measured only by the treatment of sewage, or by the recovery of rivers as living systems.
