CHENNAI: The Kosasthalaiyar sub-basin has emerged as one of the most climate-vulnerable and high-risk river systems in the Chennai basin, underscoring the fragile state of the city's water ecology amid intensifying climate stress.
The finding is part of the report, 'Towards Climate-Resilient River Systems in Chennai', released at the Tamil Nadu Climate Summit, which calls for urgent, integrated water management and adaptation measures to protect long-term water security and ecological balance.
The study categorises the Kosasthalaiyar sub-basin, along with the Cooum, as being in the "very high risk" category, based on combined indicators of hazard, exposure, and vulnerability.
The river system faces a high to very high drought risk in the run-up to 2050, even as it remains acutely flood-prone. The recent floods in Chennai exposed that the stormwater drains in highly urbanised stretches of the Kosasthalaiyar and Cooum sub-basins are unable to cope when intense rainfall overwhelms their carrying capacity.
Unplanned urban expansion has outpaced the sewage treatment capacity. This leads to the discharge of untreated wastewater into the Kosasthalaiyar, Cooum and Adyar rivers.
It has degraded surface water quality, with very high biochemical oxygen demand and faecal coliform levels recorded downstream of urban and industrial clusters. The deterioration is mirrored below ground.
A 2021 assessment across potable zones found that only 10.7 per cent of post-monsoon samples and 17.9 per cent of pre-monsoon samples met the "excellent" category for drinking.
Large swathes in the north, north-west and western parts of the basin were classified as very poor or unfit for consumption.
Elevated concentrations of total dissolved solids, calcium, magnesium, sodium, nitrate and sulphate point to cumulative contamination from industrial effluents, agricultural runoff and urban waste, signalling a profound alteration of groundwater chemistry.
Chennai master plan 2.0 and 3.0 is going to affect Kosasthalaiyar river basin. The government aggravating the situation by allowing more construction in the river basin. The plans for Chennai-Bengaluru industrial corridor and the Ponneri industrial corridor also could affect the basinNityanand Jayaraman, environment activist
The report links the basin's heightened vulnerability to degrading groundwater quality, and proximity to the coast amplifies climatic sensitivity, and socio-economic stressors such as high population density and poverty.
Hydrologically, the Kosasthalaiyar remains central to Chennai's water lifeline. Runoff from the basin, including flows from the Nagariyar, Nandiyar and Kosasthalaiyar rivers, recharges the Poondi reservoir, from where water is diverted to Sholavaram and Red Hills to meet drinking water needs.
Also, the river must maintain an environmental flow of 20 per cent of its average lean-season discharge from January to May to sustain ecological functions, a requirement increasingly under strain.
Water stress in the basin has been projected to deepen. Unmet demand, estimated at 377 million cubic metres in 2020, is expected to rise to 439 MCM by 2050 in the absence of major interventions.
The report argues that a shift towards a circular economy in water use could ease this trajectory.
Micro-irrigation across 13 per cent of cropped areas and reuse of 25 per cent of treated wastewater could reduce the unmet demand by nearly 60 per cent by mid-century.
Scaling reuse to 40 per cent could lower the deficit by about 65 per cent compared to business-as-usual projections.
The study also highlights the Integrated Urban Flood Management for the Chennai Kosasthalaiyar Basin Project as a key effort to mitigate flood risks and build resilience.
As climate pressures mount, the report stresses that coordinated policy action, infrastructure upgrades and sustainable water management are no longer optional.
It is essential to safeguard the future of the Kosasthalaiyar river system and Chennai's wider water landscape.