The waters directly behind dams tend to accumulate pollutants and have low levels of oxygenation, which can dramatically impact fish populations.
Dams are barriers erected on flowing water to create reservoirs and lakes. They are built by humans for a number of reasons, including preventing flood damage, generating electricity, and storing water, among others. Those built by humans have been around for more than 2,000 years and it is estimated that more than half of the world’s major rivers are dammed in at least one place, causing profound environmental impact. There is a lot of debate about the impact of dam construction, and one of the areas of controversy is the effect on fish and aquatic life. Dams can alter sediment patterns, obstruct migration, and affect water quality.
Dams can affect the migration rate of fish, which can expose them to predators lurking in slower-flowing waters.
When dams are built, they instantly change the body of water in which they are installed. In many cases, they are built to control the flow of water, in rivers like the Nile, for example. However, changes in currents have an effect on the aquatic ecosystem and, in general, on the environment in general, as was discovered in Egypt after the construction of the Aswan Dam.
Salmon spawn in rivers, so dams can prevent their migration.
The first thing affected by a dam is the flow of sediment, which is usually collected along stream banks and beds and carried downstream. Sediment accumulates behind the dam instead of continuing to flow with the river, and as a result, the waterbed below the dam eventually becomes rocky and cleansed of sediment. Aquatic plants cannot thrive and the nutrients in the sediments that nourish fish and other aquatic life are no longer available, sometimes causing them to die out in large numbers.
A ship passing through a lock at the Three Gorges Dam. The dam had serious effects on fish in the Yangtze River.
The most obvious impact a dam has on fish is that it obstructs migration. For some species, such as salmon, that spawn in rivers and then swim out to sea, this can be devastating. For others, not being able to go upriver means that there is not as much diversity of species. Many dams in the latter part of the 20th century were built with fish ladders and other modifications intended to allow fish passage, but many are unable to use the ladders or die in the dam machinery. This has a profound impact on fish populations.
Several species of fish native to the Colorado River are in danger due to Hoover Dam.
Dams also reduce the rate of migration, exposing sensitive fish species to predators that may hide in slower-flowing waters. In the case of salmon, navigating multiple prey and dealing with predators also affects internal biological changes that allow them to switch from freshwater fish to saltwater fish.
The quality of the water directly behind the dams is also affected, as they tend to collect pollutants along with the sediments, which must be released and somehow controlled. Especially in the case of deep structures, thermal stratification develops because there is no current to mix the water. As a result, the deepest waters are very cold and have little oxygen circulation, while the surface waters are warmer than normal. When cooler deep water is released, it can cause serious problems downstream for fish and other aquatic life, due to its poor oxygenation and the toxins it can carry.