Dams & Reservoirs

Water Education
Water Resources


Why are dams built?

There are several reasons for building dams. Dams are built to create reservoirs that capture water from streams and store the water in reservoirs. This excess water from the snowmelt and floodwaters is used throughout the year and during drought years. Dams also provide irrigation water, drinking water, hydropower, and flood control. Many reservoirs provide recreational activities like fishing, boating, and other water sports

What are dams made of?

There are several different kinds of dams. Some dams are called embankment dams. Embankment dams are called either earthfill dams or rockfill dams, depending on what material is used most in the dam. Earthfill dams are made mostly of soil, or earth. Rockfill dams are made mostly of rocks. Embankment dams are the most common type of dam in the United States. Other dams are made of concrete. Concrete dams can be either gravity dams or concrete arch dams, depending on how they are built.

How is an embankment dam built?

Before a dam can be built there are several things that have to be done. First, the location of the dam has to be chosen. Builders look for a place that is close to where the water will be used. They need to find a valley that will be able to hold the amount of water they want to store. The valley needs to be big enough, and have soils and rock that will not let all the water seep into the ground.

Geologists determine what kind of soils and rock there are by drilling holes deep into the ground and pulling out the soil. Then they test the soils to see if it will be a good place for a dam and reservoir.

Once a good location is found, engineers design the dam. They have to look at where the dam will be built to figure out how big the dam will be and what kind of dam will work best. When the design is complete, construction begins. If a dam is being built where there is already a river or stream, workers have to divert the water away from the construction site or build a small dam called a cofferdam to hold back the water while they build the dam.

Workers start building an embankment dam by digging a large trench into the existing ground that runs the length of the dam. They fill this trench with dam material to form a cutoff trench. This helps keep the water in the reservoir. Then the dam is built above ground. Engineers use different materials in different areas, or zones, so the dam will work properly. A central zone within the dam is made of clay soil that holds the water and allows very little water to get through. Other downstream zones are made of soil or rock that give strength to the dam. During construction of these different zones, workers must also build other parts of the dam like outlets and spillways

Outlets are used to control the amount of water that is let out of a reservoir at any time. The outlets have gates which can be opened and closed when the water users need them. Spillways allow water to flow out of the reservoir when it gets full so that water does not flow over the top of the dam.



When construction of the dam is complete, water is allowed to fill the reservoir so water will be available when it is needed.