There are three main types of hydropower most commonly in use:  

Storage Hydropower — Water is stored in a reservoir behind a large dam. When water is released from the reservoir, it flows through a turbine and activates a generator. The amount of electricity can be controlled on quick notice depending on demand during the day. Because of the vast quantity of stored water, this type of system can go for long periods without more water being added to the reservoir.  

Pumped Storage Hydropower — Harnessed water is cycled between a lower and upper reservoir with pumps. When electricity demand is high, water is released to the lower reservoir through turbines that produce the amount of power needed.  

Offshore Hydropower — Since seas and oceans cover 71% of earth’s surface, waves and currents contain a tremendous energy potential. While this off-shore option is still in its early stages of development, many are excited by the possibilities enabled by technology that can capture energy from the movements of the water and convert it to electricity.  

Since the water being used around the world to create hydropower is generally in great abundance (and isn’t reduced or destroyed in the process), it would seem that it’s the preferred means of power production. Not so fast--here are the top 5 pros and cons of hydroelectric production:  

PROS 

1. Environmentally Safe — As one of the most environmentally friendly forms of energy production, hydropower doesn’t use any fossil fuels, creates no harmful emissions, and provides a steady supply of clean energy. 

2. Renewable Resource — Since there are water sources basically everywhere people live, it’s widely available. 

3. Highly Reliable — Hydropower is consistently the most efficient source of renewable energy. 

4. Greatly Flexible — With the ability to control how much water runs through the turbines to activate the generators, the amount of electricity needed can be adjusted at any given time. 

5. Economical — While the investment to build a dam to create a reservoir is substantial, the costs of maintenance and upkeep are relatively low. Unlike fossil fuels, there is no cost fluctuation in naturally occurring water. On the flip side, once the reservoir is created, people can often enjoy fishing, boating, swimming, and even windsurfing.  

CONS 

1. Environmental Impact — Dams built to create reservoirs disrupt natural habitats, impact fish migration, and decrease oxygen in the water necessary for all aquatic life.  

2. Destroys Communities — Over the years, dams have displaced millions of people, permanently flooded whole towns and historic sites, and created a risk of flooding for communities living downstream. 

3. High Cost — While efficient to operate, dams can take years (sometimes decades) to build, and the cost can run into the billions. And because of their permanency, dams and reservoirs can’t be moved like solar and wind power technology. 

4. Dangerous Droughts — As our climate drastically changes, water supply can be unreliable around the globe. By restricting natural water flow, communities below the dam can have their water supply severely reduced.  

5. Limited Opportunities — With everything that goes into building a dam, reservoir, pumping stations, generators, and storage facilities, a hydroelectric project many not generate enough profit to make it worthwhile.