Dams are structures built across rivers to impound water, creating reservoirs. These human-made barriers serve various purposes, including hydroelectric power generation, irrigation, and flood control. Wetlands are distinct ecosystems where water covers the soil, or is present at or near the surface. They are highly productive environments, providing habitats for numerous species, filtering water, and mitigating floods. Despite the benefits dams offer to human societies, their construction and operation significantly threaten these ecologically rich wetland habitats.
Altered Water Flow
Dams fundamentally change the natural flow patterns that wetlands depend on. By regulating water release, dams reduce the natural flood pulses and seasonal fluctuations in water levels essential for wetland health. For instance, a study on the Nenjiang River in Northeast China found that dam construction led to a significant decline in peak discharge, flood frequency, and magnitude. This resulted in a 44% reduction in riparian wetlands along a 150 km stretch downstream of the dam.
This disruption means that wetlands may receive either consistently too much water, leading to inundation and drowning of vegetation, or consistently too little water, causing them to dry out. Natural cycles of wetting and drying are crucial for the life cycles of many wetland plants and animals, and their disruption can alter plant communities and reduce biodiversity. The altered flow regime can also weaken the ability of wetlands to regulate water flow, potentially reducing their effectiveness in alleviating floods and supporting baseflow during dry periods.
Disrupted Sediment and Nutrient Dynamics
Dams act as barriers that trap sediments and nutrients, preventing their natural flow downstream to wetlands. Rivers naturally carry nutrient-rich sediments, which are vital for replenishing and maintaining the fertility of wetland areas. When dams block this transport, the downstream waters become “hungry waters,” meaning they have a reduced sediment load and increased erosive power.
This can lead to the erosion of riverbeds and wetland areas, depriving them of the natural building blocks necessary for plant growth and habitat maintenance. Reservoirs can trap substantial amounts of essential nutrients, such as biogenic silica, reactive phosphorus, and total nitrogen. This significant reduction in nutrient supply alters nutrient cycling and can lead to a decline in plant biomass, affecting the entire food web within wetland ecosystems.
Changes in Water Chemistry
The impoundment of water by dams also alters the chemical properties of the water released downstream, creating unsuitable conditions for wetland species. Reservoirs can lead to changes in water temperature; for example, water released from deep reservoirs is often colder than natural river temperature, while surface releases can be warmer due to increased solar exposure. These thermal changes can stress or eliminate native aquatic species adapted to specific temperature ranges.
Dams can reduce dissolved oxygen levels, particularly in stratified reservoirs where deeper waters become anoxic. When this oxygen-deprived water is released downstream, it can be detrimental to aquatic life, as most species require sufficient dissolved oxygen for respiration. Changes in nutrient concentrations can also occur, sometimes leading to nutrient imbalances that promote harmful algal blooms, further reducing water quality and making it difficult for sensitive species to thrive.
Impacts on Wetland Species
The combined effects of altered water flow, sediment deprivation, and changes in water chemistry result in significant habitat degradation, fragmentation, and loss for a wide array of wetland species. Many fish species, such as salmon and sturgeon, are migratory and rely on free-flowing rivers to access spawning and feeding grounds; dams block these pathways, interrupting their life cycles and leading to population declines. The changed water flow can also reduce breeding grounds for migratory fish, which are a food source for wetland birds like egrets.
Wetland plant communities are also severely affected, with altered hydrological regimes and sediment supply leading to shifts in species composition or even the replacement of wetland communities by upland vegetation. The reduction in natural flooding can cause wetlands to shrink or disappear entirely, transforming them into different habitat types. Overall, these changes contribute to a significant reduction in biodiversity within wetland ecosystems, impacting birds, amphibians, and other wildlife that rely on these unique environments for survival.