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The Nilwala River, located in Sri Lanka’s Southern Province, winds through the heart of the Matara district, serving as a crucial water source for agriculture, fisheries, and drinking water. Stretching over 72 kilometres, it is the third-largest river in the province and is renowned for its rich biodiversity. The river supports various flora and fauna, from the mangrove-lined banks to the vibrant aquatic ecosystems that thrive in its waters.
However, climate change’s impacts are disrupting this ecosystem’s delicate balance. Rising temperatures, unpredictable rainfall patterns, and increased flooding are not only transforming the landscape but also altering the distribution of both plant and animal species. The mangroves, which once provided crucial habitats and natural flood defences are under stress, while fish populations and other aquatic life face new threats as water temperatures fluctuate.
The challenges the Nilwala River faces symbolise the broader environmental changes impacting Sri Lanka and other climate-vulnerable regions worldwide. As waters rise and wildlife shifts, the communities that depend on this river must adapt to ensure its survival and their own.
Rising Waters, Shifting Wildlife
One of the most dramatic impacts of climate change on the Nilwala River has been the increase in flood events, which has altered the habitats of aquatic and terrestrial species. Saltwater crocodiles (Crocodylus porosus), once confined to the river’s natural boundaries, are increasingly venturing into human settlements due to heavy rains and flooding. During the 2017 floods, a giant 15-foot-long crocodile was found in a tea estate, far from its typical habitat, underscoring how severe flooding is forcing wildlife to relocate.
Herpetologist Anslem de Silva, who has studied the Nilwala River for decades, said to Mongabay that crocodiles often move to calmer waters or dry land during floods to avoid swift currents and debris. “Crocodiles like calm waters, so they try to move away from rapid flows… bringing debris and logs that can bang against their bodies,” de Silva explained. These relocations often lead to increased human-wildlife conflict, with crocodiles occasionally trapped in temporary water holes or even within residential areas. Read here
Rising floodwaters are affecting crocodiles as well as many bird species, amphibians, and small mammals. Floods destroy nests, burrows, and feeding grounds, forcing them to abandon their habitats. The increased frequency of these extreme weather events is putting pressure on the ecosystem, forcing species to adapt or relocate in ways that disrupt the river’s natural balance.
Impacts on Water Quality and Aquatic Biodiversity
Recent studies show that human-induced climate change is unavoidable, adversely affecting water quality and aquatic life. The Nilwala River Basin (NRB), known for its high annual rainfall, is experiencing drastic streamflow fluctuations that alter the river’s behaviour. As rainfall intensifies, floods and droughts have become more frequent, disrupting aquatic habitats and the livelihoods that depend on them. Read here
Aquatic life in the Nilwala River also feels the effects of climate change. Fluctuating water temperatures, oxygen levels, and streamflow patterns threaten fish populations and other aquatic species. As rainfall patterns shift, the river experiences extended droughts and severe floods, each of which affects aquatic biodiversity in unique ways.
During droughts, the river’s water levels drop, increasing the concentration of pollutants and reducing oxygen availability for fish and other aquatic organisms. Our research on the Nilwala River has shown that key water quality indicators, such as Chemical Oxygen Demand (COD) and Biological Oxygen Demand (BOD), fluctuate significantly with rainfall. During dry periods, water quality declines, stressing fish populations that depend on consistent oxygen levels for survival and reproduction. Read here and here
Conversely, during heavy rains, floods wash pollutants into the river from surrounding areas, further degrading water quality. The increased intensity of floods can also disrupt spawning grounds and aquatic habitats, making it difficult for species to reproduce and thrive. The complex interplay between rainfall, water flow, and marine ecosystems highlights how vulnerable the Nilwala River is to the effects of climate change.
Changing Plant Distribution
The Nilwala River Basin is home to a wide variety of plant species, many of which are highly sensitive to water availability and temperature changes. With more frequent floods and droughts, plant communities along the riverbanks are being reshaped. Floods erode soil and uproot vegetation, while droughts limit the water availability necessary for plants to grow. Variations in river hydrology have caused changes in the areas suitable as mangrove habitats and, thus, have resulted in an altered distribution. However, increases in mangrove areas can mask the degradation of the site in terms of floristic composition, significance of the species, and biodiversity (this effect is termed “cryptic ecological degradation”). Read here
Riparian vegetation, which plays a crucial role in stabilising the riverbanks and providing habitat for wildlife, is particularly affected. When the river floods, these plants are often swept away, leaving the banks vulnerable to erosion. Additionally, certain plant species that thrive in wetter conditions are encroaching into areas where drier species once dominated, further altering the landscape.
Watch here
As plant species’ distribution changes, so does the entire ecosystem. Plants that provide animal food and shelter may no longer be available, forcing species to migrate in search of new habitats. This shift disturbs the balance of the ecosystem, affecting not only the biodiversity of the Nilwala River but also the surrounding forest and agricultural lands.
The Human-Crocodile Conflict
Sri Lanka is home to two species of crocodiles: the saltwater crocodile (Crocodylus porosus) and the mugger crocodile (Crocodylus palustris). With an estimated population of 2,500 to 3,500 saltwater crocodiles, this species—known locally as geta kimbula—is the largest reptile on Earth, capable of reaching lengths of over 6 meters. While the mugger is more commonly distributed across the island, saltwater crocodiles inhabit estuaries and river systems along the southern, western, and eastern coasts.
One of the most visible impacts of climate change on the Nilwala River is the increasing human-crocodile conflict. As floodwaters push crocodiles closer to human settlements, residents of the Matara District face a growing threat. Wildlife officials regularly respond to reports of crocodiles entering paddy fields, tea estates, and even homes. Although no major attacks have been reported in recent years, the fear of encounters remains constant.
Shehan Pathirana, a local resident, expressed the concerns of many in the region: “The Nilwala River used to be a lifeline for people living in the tiny villages. But due to the increased threat of crocodile presence, people have stopped accessing forest resources.” This ongoing fear, compounded by the increasing frequency of floods, has disrupted traditional ways of life, making it harder for communities to engage in activities that once depended on the river.
The growing human-crocodile conflict is emblematic of how climate change is reshaping relationships between wildlife and humans. As flood patterns become more erratic and unpredictable, wildlife is forced out of its natural habitat, escalating tensions between conservation efforts and community safety.
Conclusion: A Call for Sustainable Solutions
The impacts of climate change on the Nilwala River are undeniable: rising temperatures, unpredictable rainfall patterns, and increased flooding are reshaping the distribution of plant and animal species, disrupting ecosystems, and posing new threats to local communities. Addressing these challenges requires the adoption of sustainable water management practices tailored to the river’s evolving conditions.
One potential solution is the construction of water storage facilities in the upper Nilwala River Basin. These could play a crucial role in mitigating flood risks and ensuring a reliable water supply during drought periods. Such infrastructure would help regulate the river’s flow, balancing the extremes of both flood and drought that have become increasingly common.
In addition to managing water flow, ongoing efforts to monitor water quality and biodiversity are essential. By integrating physical and chemical data into a comprehensive Water Quality Index (WQI), researchers and policymakers can gain a clearer understanding of how climate change is impacting the river’s overall health. This data-driven approach would allow for more informed decisions, helping to safeguard the river’s ecosystems and the communities that rely on them.
The future of the Nilwala River, like many ecosystems worldwide, is uncertain in the face of climate change. However, through collaborative efforts, innovative approaches, and a commitment to sustainability, there is hope. By taking proactive measures today, we can ensure that the Nilwala River continues to support both its rich biodiversity and the livelihoods of the people who depend on it for generations to come.
References:
https://iwaponline.com/wpt/article/19/6/2352/102136/Impact-of-rainfall-on-the-water-quality-of-a
Up close with the Saltwater Crocodiles of the Nilwala River
As Sri Lanka floods swell with climate change, so does human-crocodile conflict
Forest Stream of the Nilwala River Basin, Sri Lanka
https://www.bbc.com/news/av/world-asia-37893973
https://www.sciencedirect.com/science/article/pii/S0960982205001624
Images/ Video:
Photos: Up close with the saltwater crocs of Sri Lanka’s Nilwala River
Banner image: https://biotope-aquarium.info/forest-stream-of-the-nilwala-river-basin-sri-lanka/
