What will happen to the environment if a dam is built?

Answer

A dam was built in Canada years ago. And one day the controller thought that the water level was too high so he let the water run out into the river that had been dammed.

It just so happened that a large group of migrating cariboo wanted to cross that same river that same day. Needless to say the water level was so high in the river that all the cariboo drowned and never made it to the other side.

So my answer is: Maybe nothing bad will happen to us, but maybe a lot of bad things will happen the animals near the dam.

Obviously, a large area is submerged when a dam is built, and this may be home to unique and endangered flora and fauna. Water which flows from hydroelectric dams is from the bottom of the dam where it is cold, dark, low in oxygen, and almost lifeless. This affects the ecosystem for many miles downstream.

Benefit

The reservoir created by a dam, while taking away some terrestrial habitat, can improve wetland, shoreline, and aquatic habitat.

Another perspective...

I have not heard of any examples of reservoirs improving wetlands except maybe in arid climates, but then many types of wetlands do not naturally belong in arid climates. Also, they do not create aquatic habitat so much as just change it from a naturally flowing wild river to a lake, which is a completely different water body and supports a completely different ecosystem.

Dams do provide more shoreline probably, and also help control water supply, generate electricity, improve agriculture, help in flood control, and provide additional water contact recreational opportunities.

However, dams are, by and large, damaging to the environment. They disrupt the natural flow regime of a river, disconnecting the river from its flood plain. The previous commenter mentioned dams improving wetlands. That may be true around the reservoir, but usually, all the wetlands wetlands below the dam suffer and often disappear completely.

We like to think that reservoirs help to improve water supply and aid in flood control, but sometimes, dams can have exactly the opposite effect. In many years, average rainfall will result in our ability to operate dams to their full effect, capturing more the snowmelt, reserving water for increased demand later in the year, controlling potential flooding from heavy rainstorms, etc. And while engineeers have done a lot of good for us, I have zero confidence in the ability of engineers to overcome nature in the long run. A river and its wetlands are already designed to deliver exactly the amount of water for natural communities in the watershed. Wetlands have numerous functions essential to healthy riverine processes. They serve as habitat for aquatic and terrestrial wildlife, serve as nurseries for young fishes and amphibians, and help filter out pollutants. They also act like sponges, absorbing water during the spring melt and releasing it back to the river later in the year when the river requires more water. All of those ecosystem functions may be taken away when a dam is built.

In addition, natural communities in the river are adapted to river life. Dams interrupt those communities. Dams are responsible for endangering numerous species, including salmon and freshwater mussels. They are hurting salmon by impairing the ability of adult salmon to swim up their home rivers to spawn, and also impede the ability of spawned salmon fry to swim downstream as they migrate toward the ocean. Also, as a family, the Unionidae (freshwater mussels) are probably the most endangered family of any in the animal kingdom. Most freshwater mussels require flow, an some exist in very localized populations. So when a dam is built and suddenly there is no flow, whole species of mussels may go extinct, and many probably already have.

Unintended consequences

Many dams have been built to control the flow of rivers. Recent flooding in the midwestern United States is thought to be caused not by climate change but the fact that the dams have allowed silt to build up and, in effect, have shallowed the riverbed. Periodic natural flooding creates a deeper channel and lessens the severity of future flood events, and this is not possible with the system of dams the way it is.