- Tamil Nadu is converting a coastal wetland in Chennai into a reservoir. Called Mamallan reservoir, it will be the city’s proposed sixth drinking water source.
- Naturalists and conservationists say the project’s environmental assessment raises questions about its claims on flood mitigation, groundwater recharge and ecological impacts.
- The site is a critical habitat for migratory birds, which will be impacted if the landscape changes.
- Fishing communities who depend on the wetland also worry about the project’s impact on their livelihoods.
Narayanan R. stood on the muddy bund of the Great Salt Lake, watching a group of flamingoes pick their way across the shallows. He remembered standing in the same spot as a 10-year-old, knee-deep in water — the day his father showed him how to fish. “The flamingoes may not come next season,” he said. “These pools will dry soon.
A few hundred metres away, Gowriamma waded through the shallow waters of the lake, her fingers sifting through the mud for shrimp. “Everything depends on these small pools. We know this water through the mud and the thorns.”
Located between Nemmeli and Kovalam in Chennai, the Great Salt Lake also known as Kovalam-Nemmeli backwaters, is a coastal wetland that spans 5,000 acres. The Tamil Nadu government plans to convert this coastal wetland into Chennai’s sixth drinking water reservoir: the Mamallan reservoir.
Chennai currently supplies between 700 and 800 million litres per day (MLD) of water. The demand already exceeds 1,100 MLD and is projected to cross 2,500 MLD by 2035, according to the project’s environmental impact assessment (EIA). The Mamallan reservoir aims to supply 170 MLD for the city’s needs. The reservoir is designed to store 1.65 thousand million cubic feet (TMC) in a single filling and an annual storage of 2.25 TMC.
However, for Narayanan, Gowriamma and many fishing families in the area, losing the Great Salt Lake means losing the only world they have ever known. “Who are we to change the contours drawn by nature?” Narayanan asks.
While the wetland falls under the Tamil Nadu Water Resources Department, these backwaters have long functioned as a fishing ground, a seasonal habitat and a flood buffer.
On January 19, 2026, the foundation stone was laid for the reservoir project. Since then fishing communities in the area have been opposing and protesting the move. In May, the Tamil Nadu state elections resulted in a new government, raising hope for the communities. So, on June 16, fishing communities from the nearby villages gathered at the Kovalam fish market to oppose the project and called for the new government to scrap it. But the project remains underway.
A landscape built by tides
Drive about 35 kilometres south from Chennai on the East Coast Road and you reach the Kovalam-Nemmeli backwaters or the Great Salt Lake. It is not a lake in the conventional sense. It is an ecotone — a living transition zone where freshwater and saltwater meet, sustaining a unique ecology. “Twice a day, seawater from the Bay of Bengal pushes through two inlets, Muttukadu and Kokilamedu, and spreads across the wetland,” Narayanan said. “Twice a day, it recedes.”
The Buckingham Canal connected to the wetland carries surplus water from the catchment toward the sea. Mangroves and salt marshes in the area buffer the coastline from storm surges.
Nemmeli is recognised as one of 141 priority wetlands under the Tamil Nadu Wetlands Mission. Yet, over the years, the coastline has faced pressure from pollution, sewage discharge and desalination infrastructure. A 100 million litres per day-(MLD) desalination plant commissioned in 2013 caused sea erosion and salinity intrusion, according to marine conservationists. A second plant followed in 2024, and a third is under construction nearby. And now comes the Mamallan reservoir.
In February 2026, a group of 22 ecologists, hydrologists and ornithologists wrote a letter warning the then Chief Minister MK Stalin that the project could irreversibly damage a critical coastal wetland. The EIA ran for three months, from April to June 2025, missing the monsoon entirely. This is when the wetland’s hydrological character is most visible, and its migratory birds arrive, the conservationists critiqued.
Elango Lakshmanan, faculty at the Indian Institute of Technology, Madras, who has also been on monitoring committees on government projects, said, “The EIA has to be a year-long monitoring and should reference data over 50 years, to account for fluctuations in weather.”
Naturalist Yuvan Aves, part of the group that wrote the opposition letter, has surveyed birds here since 2017. He questioned the ecological basis for locating a reservoir in a coastal wetland. “The biggest conflict is between what’s written in a document and the ecological reality. There is no precedent of a reservoir by a coastal wetland,” he said.
More clarity needed on flood control
The villages west of the Old Mahabalipuram Road, adjacent to the planned reservoir site, flood during the northeast monsoon. The EIA for the project identifies three reasons for the flooding: high raised bunds on the salt pans obstruct drainage of floodwater, inadequate outlets along the Old Mahabalipuram Road, and the Kelambakkam-Kovalam Link Road blocking a major floodplain.
The reservoir project proposes to address the first issue by removing bunds and installing peripheral drains. The reservoir addresses only one of the three causes of flooding identified in its own assessment, and not the most important ones, notes a study by non-profit Suzhal Arivom, which works on environmental education.
The EIA’s risk assessment acknowledges the possibility of “overtopping, bund breach, flooding of downstream zones, or spillway failure” during heavy rainfall. Its mitigation response is limited to “proper maintenance of outlet regulators.” It also does not model how the freshwater-saltwater interface will behave if the wetland’s tidal connection is severed.
Deepak Venkatachalam, the founder of Suzhal Arivom, draws parallels with a 2021 proposal to build a reservoir in the Pallikaranai marshland, which was dropped after experts questioned its ecological viability. “If construction continues at Nemmeli, it will face the same disaster Pallikaranai narrowly avoided,” he said.
However, Vijayaraj B., the newly elected MLA for Thiruporur constituency where this project falls, said he has heard all the concerns from fishing communities. “Fishermen have warned me that obstructing waterways could cause flooding. The documents don’t clearly explain the mitigation. We have asked the Water Resources Department (WRD) for a detailed plan, with clear solutions. Once I have it, I will personally meet the people and explain.”
Coastal reservoir and groundwater recharge
For a reservoir to recharge groundwater, its bed must be above the groundwater level to allow downward flow, and it must be sufficiently permeable. The EIA notes that the reservoir site is underlain by low-permeability clay and clayey loam. Water stored in such soils generally does not percolate effectively, making groundwater recharge hydrologically challenging.
The salinity picture is equally concerning. A 2022 study published in the journal Hydrogeology by researchers from IIT Madras found seawater intrusion already extending 700 metres inland along the coastline. Sand dunes that once acted as natural recharge zones and kept saltwater at bay, have been flattened by the East Coast Road.
“If there is so much risk for salinity intrusion, what’s the point of creating a freshwater reservoir here?” asked Venkatachalam. “Water moves as per the gradient. It remembers its ways.”
The EIA’s environmental cost-benefit analysis contains only a single note: “Not recommended for scoping stage.”
“For a project that will permanently convert a coastal wetland, an EIA failed to weigh the losses against gains. This is not just a procedural oversight but a fundamental failure of the system,” said Saravanan K., a conservationist.
What the EIA left out
Thousands of people in the fishing villages of Thiruvidanthai, Nemmeli, Pattipulam and neighbouring villages depend on this wetland for their daily income. Yet the EIA mentions fishers only once, noting that construction may affect “fish and prawn populations until natural hydrology is restored.”
It does not estimate how many people depend on the wetland or how their livelihoods may change after its conversion.
Residents in the nearby villages fish in shallow tidal pools and floodplains that appear and recede with the tides, collecting shrimp, crabs and fish by hand. These brackish shallows also serve as nursery grounds for juvenile fish, shellfish and shrimp larvae used as live bait in deep-sea fishing. Field observations by Suzhal Arivom at the adjacent Muttukadu estuary, documented 13 species actively nursing juveniles in this coastal system.
“Destroying these tidal pools does not just affect a small group of fishers,” said Narayanan. “It breaks the entire breeding cycle that sustains the offshore fishery as well.”
Vijayaraj assured the WRD was working on a compensation plan for a few fishers who depend on these tidal pools for about three months, October, November, and December. “WRD will issue ID cards to the affected fishers for continued access to the wetland,” he said.
This however, isn’t the first time families here have been displaced. Several families had relocated to this coastline after the Kalpakkam nuclear plant pushed them from their homes further south, Narayanan said. “It is the same story all over again,” he added.
Impacts on birds
Every winter, thousands of birds travelling the Central Asian Flyway visit this coastline. The Wildlife Institute of India has identified the backwater site as an Important Marine and Coastal Biodiversity Area, one of only 107 nationally. The EIA recorded 102 bird species and flagged potential impacts on wading species.
Yuvan Aves explained what that means in practice. “Shallow brackish water is very ecologically rich,” he said. “Depth and salinity matter a lot for waders. Many birds are so tiny that they cannot wade in deep water. Flamingoes, too, can walk only in ankle-deep water. The proposed reservoir will raise water levels to three metres during the monsoon. The mudflats will be gone.”
“If the salinity changes,” he said, “the birds won’t come.”
The EIA proposes converting abandoned salt pans into habitat, and constructing six artificial islands within the reservoir, noting that it will act as “will act as roosting and feeding region to the aquatic birds.”
However, a study by Suzhal Arivom argues these measures are ecologically unviable, noting that migratory waterbirds often return to the same foraging sites year after year; and that salt pans are hypersaline and so cannot replicate the habitat of a wetland.
“Most species rely on specific low water levels for foraging. Shorebirds and waders are physically constrained by their leg and bill lengths; even a slight increase in depth can submerge their feeding grounds. Maintaining natural, shallow brackish waters is critical for the conservation of Nemmeli’s avifauna. Any alterations do not merely change the landscape; they may lead to rapid habitat abandonment and the hyper-localised extinction of bird populations,” notes Yuvan Aves.
“Most wetland birds exhibit strong site fidelity, as in, they return to the exact same foraging patches year after year. Also, if a bird population is displaced forcibly, it can lead to inter-species conflict and starvation.”
No alternative sites were considered
“No other alternative site examined for storage of water.” This is how the EIA addresses whether other sites were considered for water storage.
“This is a big-scale project that proposes long-term benefits for the people. So, before starting something like this, they should’ve examined the existing canals, and tanks,” said Deepak. It means explaining, with evidence, why other options were ruled out. None of this appears in the document.
A 2020 paper that critiqued EIA processes argued that EIAs should be overseen by an independent authority rather than consultants hired by the project, with communities formally represented in the process. The Supreme Court made a similar recommendation in a TN 2014 case, calling for an independent National Regulator for environmental clearances. Neither proposal has been implemented.
S. Janakarajan, president of the South Asia Consortium for Interdisciplinary Water Resources Studies, pointed to an alternative for this reservoir.
Across the nearby districts of Tiruvallur, Kancheepuram, and Chengalpattu, there are over 3,600 hydrologically connected waterbodies. “The first priority should be to fix the stormwater drains, which carry sewage, and desilting and deepening these tanks so they fill effectively during heavy rainfall,” he said. “This could significantly augment the region’s storage without bothering the coastal ecosystem. There is no way we can artificially create a water body that matches the hydrological and ecological rigour of a naturally occurring one.”
Yet Janakarajan is not convinced the wetland will survive without the project. A reservoir, he argues, may be the only administrative mechanism that prevents the land from being converted to other uses.
His argument points to the larger problem of India’s land classification system, inherited from colonial governance. It classifies land as privately owned patta land, forest land, and wasteland/poramboke. However, wetlands and fishing grounds have no such distinct classification.
Yet the EIA’s own land-use data classifies 14.68 square kilometres of the project area as coastal wetland, a category environmental courts have repeatedly directed governments to protect. The report addresses this contradiction by stating that after construction, “the entire area will be classified as a wetland region.”
The conservationists and ecologists who oppose the project proposed declaring the area as the protected Mamallan Lagoon with immediate effect. But the wetland is now being transformed, even as fundamental questions about its communities, hydrology and biodiversity remain contested.
Banner image: Many women from the Irular community depend on seasonal brackish pools around the Kovalam-Nemmeli backwaters for their livelihood. Imge by Smitha T.K.