Ipsa Tripathy
Bhubaneswar: A wetland inside a growing city can look like an empty patch of water surrounded by buildings. To an urban planner, it may appear to be undeveloped land. To an ecologist, however, it is part of the city’s natural infrastructure.
Wetlands include marshes, shallow lakes, floodplains, swamps and other areas where water remains at or near the surface for periods of time. In urban landscapes, these ecosystems can perform functions that are otherwise expensive to reproduce entirely through concrete infrastructure. They can hold stormwater, retain pollutants and sediments, provide habitat and influence local temperatures.
A natural buffer against flooding
One of the most important functions of an urban wetland becomes visible during heavy rain. Cities contain large areas of roofs, roads, pavements and other impervious surfaces. These surfaces prevent much of the rainfall from infiltrating into the ground and rapidly move water into drainage systems. A recent review of urban hydrology found that urbanisation substantially changes rainfall runoff and can increase local flood and pollution risks.
Wetlands provide somewhere for part of that water to go. Their shallow basins, soils and vegetation can temporarily store rainfall and slow its movement through the landscape. This does not mean a wetland can prevent every flood. Its effectiveness depends on its size, condition, connection to surrounding waterways, available storage and the amount and intensity of rainfall. But the basic hydrological function is well established: wetlands can regulate water flows and reduce flood risk. When wetlands are filled or disconnected from natural drainage pathways, that storage capacity is reduced.
The city’s biological water filter
Water entering an urban wetland can carry sediments, nutrients and other pollutants from roads, neighbourhoods and surrounding land. Wetland vegetation, soils and microorganisms can influence what happens to these substances. Sediments can settle when flowing water slows. Plants can take up some nutrients, while microbial processes in wetland soils transform organic matter and nutrients.
Constructed wetlands demonstrate these processes particularly clearly. A 2023 review of 335 field-scale constructed wetlands found that these systems can remove substantial amounts of organic matter and nutrients from wastewater, although their performance varies with design, climate, wastewater composition and management. Natural urban wetlands are not wastewater-treatment plants, and they should not be treated as unlimited pollution sinks. Excessive pollution can damage wetland ecosystems and reduce their ecological functions. Their ability to improve water quality therefore depends on their condition and the pollutants involved.
A refuge for urban biodiversity
A wetland can also be one of the most biologically active parts of an urban landscape. Water, wet soils and vegetation create different habitats within a relatively small area. Fish, amphibians, insects, aquatic plants and birds can use these habitats for feeding, breeding or shelter. Wetlands are recognised globally as important reservoirs of biodiversity.
For migratory birds in particular, urban wetlands can provide feeding and resting areas along migration routes. Their ecological value, however, depends on habitat quality, water conditions, surrounding land use and connectivity with other habitats. This makes the loss of a wetland more significant than the loss of an unused piece of land. Removing it can eliminate a functioning ecological system.
Cities are generally warmer than their surrounding rural areas because buildings and paved surfaces absorb and store heat. Wetlands can influence this local thermal environment through water, vegetation, shading and evapotranspiration. Research from Kolkata found measurable temperature differences around urban wetlands, with the cooling effect varying according to wetland size and landscape characteristics.
More recent research has also found cooling effects associated with urban wetland parks. A 2026 study of 143 parks in Wuhan reported that wetland parks produced greater surrounding land-surface cooling than conventional urban parks in that study area. The effect is not universal or constant. Weather, humidity, wetland size, vegetation, water availability and landscape design all influence the amount and extent of cooling.
Urban development often puts pressure on wetlands because flat, water-rich areas are attractive for construction and infrastructure. Wetlands can also become polluted by sewage, solid waste and altered drainage. The consequences extend beyond biodiversity. Losing wetland storage can alter urban water movement, while the loss of vegetation and open water can reduce local ecological and thermal functions. The Convention on Wetlands describes urban wetlands as important natural infrastructure and notes their role in flood regulation, water quality, biodiversity and climate moderation.
The science does not suggest that wetlands should replace drainage systems, wastewater treatment plants or other engineered infrastructure. Rather, evidence increasingly supports combining natural and engineered systems. A city that protects its wetlands is therefore not simply preserving scenery. It is retaining parts of a functioning water and ecological system. As cities become denser and rainfall extremes and heat become more important challenges, wetlands can provide several services from the same piece of land: storing water, supporting biodiversity, influencing water quality and moderating local temperatures. The quiet patch of water beside a road may therefore be doing far more work than the city around it suggests.