Within recent decades, the environmental impacts of dams have been debated by people all around the world. While dams do perform important functions, their effects can be damaging to the environment. People have begun to question whether the positive contributions of some dams are outweighed by those negative effects. Below shows some environmental effects from dam construction:
Stream flow
Prior to dam construction, most natural rivers have a flow rate that varies widely throughout the year in response to varying conditions.
The most common attribute of flow regulation is a decrease in the magnitude of flood peaks and an increase in low flows.
In cases where the entire flow has been diverted for other uses, there may no longer be any flow in the original channel below the dam.
By withholding and then releasing water to generate power for peak demand periods, dams may cause downstream stretches to alternate between no water and powerful surges that erode soil and vegetation, and flood or strand wildlife.
The irregular releases destroy natural seasonal flow variations that trigger natural growth and reproduction cycles in many species. Peaking power operations can also cause can cause dramatic changes in reservoir water levels up to 40 feet that can degrade shorelines and disturb fisheries, waterfowl, and bottom dwelling organisms.
The dams remove all the sediment from the river and as the sediment reaches the slow-moving reservoir above the dam, it drops out and collects on river and reservoir bottoms. The loss of sediment downstream can degrade in stream habitat and cause the loss of beach at the mouth of the river.
Barrier to migration
Many fishes must move upstream and downstream to complete their lifecycles. Dams are often built without fish ladders. When fish ladders are provided, they seldom work as needed. If enough adult fishes do manage to climb above a dam, there remains the issue of their young to get back downstream.
Fish migrating downstream or upstream can become disoriented, bruised, stressed, or mortally injured from contact with turbines or other parts of the facility. Many are killed by predators while they wander, lost, in the reservoir above the dam. Many are killed in their fall downward through the dam to the river below. They aren't killed by the fall itself, but by the high levels of nitrogen gas at the base of the dam.
The most commonly cited species affected by the presence of dams is the salmon. Salmon have been isolated from their spawning streams by impassable or dams with fish ladders, but with only limited success as discussed above. One of the examples is the steep decline in salmon populations in the Pacific Northwest and California is perhaps the best known negative environmental impact associated with hydroelectric.
Fish ladders
Water quality impact
Water storage in reservoirs induces physical, chemical and biological changes, all of which affect water chemistry. Consequently, nutrients, particularly phosphorous, are released biologically and leached from flooded vegetation and soil.
Water with low dissolved oxygen (DO) levels released to the river downstream may harm aquatic habitat in the river and contribute to other water quality problems. Dissolved oxygen is necessary for healthy aerobic activity in aquatic ecosystems. The presence of the dam will disrupt the natural processes of aeration (the movement of water) and diffusion, ways oxygen dissolves into water. Water will move more slowly downstream, thus making it more difficult for available oxygen to be present in the water. This could have a negative impact on the aquatic ecosystem downstream.
Decomposition of organic matter and the leaching of mercury from the soil can introduce toxins.
However, water sometimes passes over a spillway, rather than through the turbines. As water plunges into the pool at the base of the dam, too much air can be trapped in the water, creating "gas supersaturation" a condition that in some fish species fosters something called lethal gas bubble disease.
Surface temperatures in the reservoir may rise when the flow of the water is slowed. If water is released from the top of the dam, this warmer water may increase river water temperature downstream. Cooler downstream temperatures may result when cool water is released from the bottom of a reservoir. Such altered conditions can affect the habitat, growth rate, or even the survival of fish and other species.
Reductions in water quantities can increase salinity and make the water unusable for drinking and irrigation.
(d) Resettlement
As people normally settle along rivers, where water for drinking, irrigation, power, and transport are readily available, reservoir flooding can displace huge populations.
The relocation of potentially large human populations happens if they are constructed close to residential areas such as the construction of Three Gorges dam built in China which forced over a million people to relocate.
(e)Starving the River
Dams hold back not only sediment, but also debris, which would be the constant feeding of the river for the life of organisms (including fish) downstream.
This debris includes leaves, twigs, branches, and whole trees, as well as the organic remains of dead animals.
Debris not only provides food, it provides hiding places for all sizes of animals and surfaces for phytoplankton and microorganisms to grow.
Adding to the problem, although debris might come from the river above the dam, it is instead trapped in the reservoir, and never appears downstream.
The bottom level of the food web is removed. All in all, the loss of sediment and debris means the loss of both nutrients and habitat for most animals.
