Why are Dams Potentially Harmful to Fish?

Dams are barriers erected in flowing water to create reservoirs and lakes. They are constructed by humans for a variety of reasons, including to prevent flood damage, to generate electricity, and to store water, among others. Ones that humans built have been in existence for over 2,000 years, and it is estimated that over half of the world's major rivers are dammed in at least one place, causing a profound environmental impact. There is a great deal of debate over the impact caused by damming, and one of the areas of contention is the effect on fish and aquatic life. Dams can change sediment patterns, obstruct migration, and affect the quality of water.

When dams are constructed, they instantly alter the body of water they are installed in. In many instances, they are built to control the flow of water, in rivers such as the Nile, for example. Changing currents have an effect on the aquatic ecosystem, however, and usually on the wider environment in general, as discovered in Egypt after the construction of the Aswan Dam.

The first thing to be affected by a dam is the flow of sediment, which is usually picked up along banks and stream beds and carried downstream. Sediment collects behind dam, rather than continuing to flow out with the river, and as a result, the water bed below the dam eventually becomes rocky and scoured clean of sediment. Aquatic plants are unable to thrive, and the nutrients in the sediment that nourish fish and other aquatic life are no longer available, sometimes causing extinction in large numbers.

The most obvious impact that a dam has on fish is that it obstructs migration. For some species, such as salmon, which spawn in rivers and then swim out to sea, this can be devastating. For others, being unable to travel up the river means that there is not as much species diversity. Many dams in the latter part of the 20th century were built with fish ladders and other modifications designed to allow the passage of fish, but many are unable to use the ladders or die in the machinery of the dam. This has a profound impact on fish populations.

Dams also decrease the rate of migration, exposing sensitive fish species to predators that may lurk in slower flowing water. In the case of salmon, navigating multiple dams and contending with predators also affects the internal biological changes that allow them to switch from being freshwater fish to saltwater fish.

Water quality directly behind dams is also affected, because they tend to collect pollutants along with sediment, which must be released and controlled in some way. Especially in the case of deep structures, thermal stratification develops because there is no current to mix the water. As a result, the deeper water is very cold and has poor oxygen circulation, while the surface water is warmer than usual. When colder 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 may carry.

Dams can severely disrupt fish migration by creating barriers that prevent species from reaching their traditional breeding or feeding grounds. For instance, salmon are well-known for their upstream migrations to spawn, but dams can block their path, leading to population declines. Fish ladders and bypass systems are sometimes used to mitigate this, but they are not always effective.

Altered water flow from dams can change river temperatures, sediment transport, and the timing of natural flow events, which are crucial for fish life cycles. According to the World Wildlife Fund, such changes can affect spawning cues and the availability of suitable habitat, leading to reduced fish populations and biodiversity loss.

Dams can lead to poorer water quality by reducing the river's ability to cleanse itself, causing oxygen levels to drop and pollutants to accumulate. This creates a hostile environment for fish, which can suffer from increased stress, disease, and mortality. The stagnant water in reservoirs can also lead to algal blooms that further degrade water quality.

The construction of dams has been linked to the decline and even extinction of fish species. The interruption of natural river ecosystems can eliminate critical habitats and disrupt food chains. According to the International Union for Conservation of Nature, numerous freshwater fish species are at risk due to habitat alteration from damming.

Dams can isolate fish populations, leading to reduced genetic diversity as the separated groups are unable to interbreed. This loss of genetic exchange can make fish populations more vulnerable to diseases, environmental changes, and reduce their ability to adapt to new conditions, potentially threatening their long-term survival.

To reduce the impact of dams on fish, measures such as fish ladders, bypass channels, and dam removal have been implemented. Environmental flow releases can mimic natural river conditions, and sediment management can help maintain habitat downstream. Ongoing research and monitoring are essential to improve mitigation strategies and ensure the health of aquatic ecosystems.