Hydroelectric power is the process of converting the kinetic energy of flowing water into electricity. While it is often promoted as a renewable and clean source of energy because it does not burn fossil fuels, its construction and operation fundamentally alter the natural environment. Building a dam and creating a reservoir imposes profound physical, biological, and atmospheric changes on the river ecosystem and the surrounding landscape. Understanding these impacts is necessary for a balanced view of hydropower’s role in the global energy mix.
Alterations to River Flow and Sediment Transport
The construction of a dam drastically changes the natural flow regime of a river. Hydropower operations, especially those designed for “peaking power” to meet high electricity demand, can cause rapid fluctuations in downstream water levels. This sudden, irregular flow can scour riverbeds, destabilize banks, and strand aquatic organisms adapted to stable or predictably changing conditions.
A major physical consequence downstream is known as the “hungry water” effect, resulting from sediment starvation. The dam traps the river’s natural load of sand, silt, and gravel in the reservoir upstream. The water released downstream, deprived of its sediment, has an increased capacity for erosion by scouring the riverbed and banks. This incision lowers the riverbed, which can undermine infrastructure like bridges and lower the local water table, affecting riparian vegetation and groundwater access.
Dams cause thermal pollution by altering the natural temperature profile of the river. If water is released from the deep bottom layer of a stratified reservoir, it is often unnaturally cold for the downstream environment. Conversely, water released from the surface layer of a shallow reservoir can be warmer than the river’s historical temperature. These temperature shifts can disrupt the metabolic rates, growth, and reproduction of temperature-sensitive aquatic species.
Reservoir Creation and Terrestrial Habitat Loss
Creating a hydropower reservoir requires the permanent flooding of large tracts of land upstream of the dam structure. This inundation destroys pre-existing terrestrial ecosystems, including forests, wetlands, and agricultural land, leading to an immediate loss of habitat for flora and fauna. The flooded area represents a permanent change in land use, shifting the landscape into a deep, still-water lake environment.
The resulting reservoir creates a significant barrier to the movement of land animals. This body of water fragments the remaining terrestrial habitats, isolating animal populations on opposite sides of the reservoir or on newly formed islands. Habitat fragmentation restricts genetic exchange and makes smaller, isolated populations more vulnerable to local extinction events.
The displacement of wildlife and the destruction of riparian zones contribute to a broader decline in the ecosystem’s overall health. Furthermore, the creation of reservoirs has historically led to the displacement of human communities and the inundation of sites of cultural or historical significance.
Impact on Aquatic Organisms and Migration
The dam is a physical barrier that severely impacts the life cycles of migratory aquatic organisms, such as salmon, sturgeon, and eels. These species rely on longitudinal connectivity, needing to move between different parts of the river system for spawning, feeding, and rearing. Blocking these migration pathways can lead to sharp population declines and even local extinction.
Engineers often install fish passage facilities like fish ladders or lifts to mitigate this barrier effect. However, the effectiveness of these structures is highly variable and often limited, as they may not be suitable for all species. Navigating these systems expends significant energy for migrating fish, which can reduce their reproductive success or survival.
The reservoir dramatically alters water quality, creating new biological stresses for aquatic life. Decomposition of organic matter on the bottom can deplete dissolved oxygen levels, leading to hypoxic or anoxic zones. When oxygen-poor water is released downstream, it can create “dead zones” where native species cannot survive.
The long residence time of water in the reservoir favors the proliferation of lentic species, which often outcompete native riverine species. This shift can foster the dominance of non-native or invasive species. Additionally, juvenile fish migrating downstream can be injured or killed by being forced through the dam’s turbines or spillways, a process known as entrainment.
Greenhouse Gas Emissions from Reservoirs
While hydropower is often labeled carbon-neutral, the reservoirs themselves can be a source of atmospheric greenhouse gas emissions. When terrestrial biomass is flooded during reservoir creation, this organic material begins to decompose. This decomposition occurs primarily in the anoxic (oxygen-depleted) conditions at the bottom of the reservoir.
Anaerobic decomposition releases potent greenhouse gases, most notably methane (\(\text{CH}_4\)). Methane is a powerful warming agent, with a much greater immediate impact than carbon dioxide (\(\text{CO}_2\)). These emissions are particularly high in tropical reservoirs, where warmer water temperatures accelerate the decay of organic matter.
Methane is released into the atmosphere through diffusion from the water surface and significant bubbling from the reservoir bottom. Methane is also released when water is discharged through the turbines and spillways, particularly when the intake draws from the oxygen-poor bottom layer of the reservoir.