Dams are barriers erected in flowing water to create reservoirs and lakes. Dams are constructed by humans for a variety of reasons: to prevent flood damage, to generate electricity, and to store water, among others. Human built dams 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.

When dams are constructed, they instantly alter the body of water they are installed in. In many instances, dams are constructed to control the flow of water, in rivers such as the Nile, for example. However, changing currents have an effect on the aquatic ecosystem, 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 dams, 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 fish species, such as salmon, which spawn in rivers and then swim out to sea, this can be devastating. For others, being unable to pass dams 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 fish 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 dams tend to collect pollutants along with sediment, which must be released and controlled in some way. Especially in the case of deep dams, 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 from dams, it can cause serious problems downstream for fish and other aquatic life due to its poor oxygenation and the toxins it may carry.