How a river dies: Thenpennai, Tamil Nadu's Second-Longest River, Is Killing Crops, Fish, And Is Too Polluted To Use

Centuries ago, it was celebrated in Sangam literature, depicted as flourishing with lush vegetation along its banks. It was worshipped too, with temples built along its banks. Today, the 497 km Thenpennai, Tamil Nadu's second-longest river after the Cauvery, flows frothy and foul.
"It used to nourish our fields," says farmer K Mariappan. "Now it kills our crop." He recalls how his father and grandfather harvested an abundance of groundnuts thanks to the clean waters of Thenpennai. "Now, crops wilt when irrigated with the polluted water. A decade ago, I harvested 300 quintals of cabbage per acre. Now it's down to 140. Even that's hard with the chemical foam."
Like the Cauvery, the Thenpennai is an interstate river, rising from the Chennakesava Hills in Karnataka's Chikkaballapur district and winding its way through Bengaluru's industrial corridors before entering Tamil Nadu near Hosur.
In Tamil Nadu, it passes through Krishnagiri, Dharmapuri, Tiruvannamalai, Villupuram, and Cuddalore, its watershed spanning nearly 3,700 sq km.
In Krishnagiri, the Kelavarapalli dam supports nearly 8,000 acres of farmland. Further downstream, the older Krishnagiri Reservoir Project provides water to more than 9,000 acres across 16 villages in Krishnagiri. For most of the year, the river runs dry, flowing only during the monsoons.
When it does, it raises the water table and replenishes tanks and reservoirs. But, say farmers, over the past few years, when water is released from Kelavarapalli dam, it is full of effluents that inundate farmland, with the foam sometimes reaching as high as the branches of trees.
Tests have found high levels of industrial pollutants, including detergents and chemicals that reduce dissolved oxygen and harm aquatic life.
In May 2024, Thenpennai foamed white with toxic froth, and farmers in Krishnagiri urged the state govt and district administration to tackle the problem. A month later, on June 9, a 15-member team from the Central Water Commission arrived at Kelavarapalli dam to assess the damage. "They took water samples," says Poomozhi, an environmentalist from Salem. "But no action has been taken." He says a Central Pollution Control Board inspection in 2020 found the pollution was due to untreated effluents from Bengaluru-based industries.
"The committee recommended restoring Bellandur and Varthur lakes, but nothing has been done."
Each day, 30 million litres are drawn from the river by the Hosur municipal corporation for drinking water, and another 13 million litres supply SIPCOT industries. The Sathanur Dam, at Tiruvannamalai, with a capacity of 7.3 TMCFT, is the largest on the river. As a result, much of Bengaluru's sewage reaches Thenpennai via Bellandur, Varthur, and other channels.
Tests revealed coliform levels ranging from 2.42 lakh to 19,863 MPN/100 ml, far above the permissible 2,500 MPN/100 ml for bathing.
"How can villagers be expected to drink this water?" says Poomozhi.
A few years ago, researchers from IIT Madras identified high concentrations of emerging contaminants in the Cauvery, detecting 15 pharmaceutical pollutants, including anti-inflammatories (like ibuprofen), stimulants (such as caffeine), antidepressants, and antibiotics.
Some of these were found at levels several times higher than in other rivers across India. The river was also contaminated with plastics and heavy metals.
The study recommended regular monitoring and upgrades to wastewater treatment systems to curb the pollution.
Last month, Krishnagiri collector C Dinesh Kumar inspected the site and assured residents that the state would approach the National Green Tribunal (NGT). "Steps are being taken to form a committee for evaluating the river and the nearby industries." But farmers say they've lost hope and fear the Thenpennai is headed the way of the Noyyal, a tributary of Cauvery that was choked by effluents from textile dyeing units in Tirupur.
Once a vital water source for Coimbatore, the Noyyal is now often described as a "gutter" clogged with plastics and sewage.

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