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Nilwala woes: Saltwater intrusion leaves Matara thirsty
View(s):By Malaka Rodrigo
For generations, the Nilwala River has been Matara’s lifeline, quenching the thirst of its people and nourishing its paddy fields. But this week, that lifeline faltered as saltwater surged upstream, crippling the main pumping stations and leaving residents scrambling for water.
“This is the first time in years we’ve had to bring pots and bottles to collect water from bowsers,” said a resident waiting at a tanker sent by the National Water Supply and Drainage Board (NWSDB).
Saltwater intrusion in the Nilwala is not new. Normally, the river’s flow pushes seawater back to the ocean. But in dry weather, when river discharge drops, high tides push brackish water inland, contaminating intake points.
The salinity barrier built across the Nilwala River and the bypass canal, on the right
To tackle this, engineers proposed a salinity barrier in 2003. Construction began in 2012, and the barrier was commissioned in 2022. Designed to block tidal surges of seawater, it was hailed as a solution. But farmers and upstream residents soon alleged the barrier was causing unprecedented flooding, as was experienced in 2023, when heavy rains left villages submerged for weeks.
Mounting pressure, including protests and political backing, forced the authorities to dig a bypass canal beside the barrier to ease flooding. But that canal has now turned into a conduit for seawater.
“The bypass canal is one-third the size of the river, so at high tide, seawater flows right past the salinity barrier,” Samantha Kumara, an NWSDB engineer overseeing the southern region, told the Sunday Times. 
“The pumping stations cannot process salty water without damaging equipment and risking public health. When salinity levels rise, we have no choice but to stop pumping,” he said.
On average, about 11,000 cubic metres of water are pumped daily to meet Matara’s needs. But with the Nadugala intake now compromised, supply has been disrupted.
Unlike the Kelani or Mahaweli, the Nilwala is short and low-lying, meeting the sea just a few kilometres downstream at Matara. That geography makes it acutely vulnerable to tidal influence—an effect now worsened by rising sea levels and erratic rainfall linked to climate change. “It amplifies the reach of saltwater inland,” explained Deepika Thrimahavithana, former Southern Region director of the Irrigation Department.
She also noted that during construction, engineers had to block half the river, which did contribute to upstream flooding, and the government compensated farmers for those losses. “But after the construction finished, these blockages were removed, and it is unfair to blame every flood on the barrier,” Ms. Thrimahavithana said, noting that the design was even upgraded from three floodgates to five.
Recent floods, according to hydrogeologist Moditha Hiranya Kodikara-arachchie, exceeded the design’s return period, and he argued that other factors, such as illegal landfilling of the Nilwala floodplain, land-use changes in the upper catchment, and encroachments on natural drainage, intensified the flooding. “This entire stretch up to Godagama is a floodplain, and it will flood from time to time, and only proper settlement planning can reduce the impacts,” he said.
Officials also discovered piles of sandbags left upstream of the barrier, which may have worsened the 2023 floods. These were removed in December 2024, and the authorities believe the removal will reduce future risk.
As the crisis deepened this week, the military and Disaster Management Centre moved to block the bypass canal, stopping seawater from slipping past the barrier. By Friday night, the water supply had resumed, giving residents some relief. But many remain worried. “We need a permanent solution,” they say.
However, the environmental activists argue the problem lies in where the barrier was built.
“The original plan was to set it further upstream. If that had happened, we wouldn’t be facing this issue,” said activist Jayathissa Kulathunga, accusing the authorities of shifting the site to continue using the Kadduwa intake. “The current barrier is just four kilometres from the river mouth. That slows the flow and increases flood risk during heavy rains.”
Computer models back some of these concerns. Shashika Iresh, an engineer of the Irrigation Department, said simulations show the barrier could raise upstream water levels by as much as 30 centimetres during high flows, far above the five centimetres predicted in feasibility studies. “There seems to be an issue that must be addressed,” he admitted.
“But the solution must be based on science, not on political or other agendas,” Mr. Iresh emphasised. “All stakeholders must come together to find a balance ensuring Matara gets clean water while easing upstream flooding. That is the only sustainable way forward.”
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