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Three types of power stations can be found (DG ENV/ARCADIS & Ingenieurbüro Floecksmühle 2011):

• Hydropower stations with storage reservoir. A storage reservoir offers the opportunity to store energy and to meet e.g. the peak electricity demands. Such reservoirs can comprise daily, seasonal or yearly storage. Many of the large HP stations operate with a reservoir.
• Run-of-the-river stations. This type of installation uses the natural flow of a water course in order to generate electricity. There is no intention to store water and to use it later on. This type is most common for small hydropower stations but can also be found with large stations.
• Pumped storage hydropower plants. Pumped hydropower stations utilize two reservoirs located at different altitudes. Water can be pumped from the lower into the upper reservoir and can be released, if needed, to the lower reservoir producing energy on ts way through the turbines. In times of high demand e.g. during peak hours electricity is produced to satisfy the demand. When there is a surplus of electricity in the system, water can be pumped to the upper reservoir.

The different types of hydropower plants have different effects on the ecosystems and hydromorphology. Generally the hydropower plants with storage reservoirs generate more severe impacts on the river system including loss of connectivity, change in water flow regime and reduced sediment flow. There are unfortunately no European overview of the number of hydropower plants by types and their location. Generally the reservoir type hydropower plants are found in the mountainous areas with steep relief, while the larger run-of-the-river stations are found on the main course of larger rivers and their tributaries. Smaller hydropower plants are often found on relative smaller rivers and with limited storage, but often acting as migrating barriers. However, compared to the impacts generated per electricity production the impacts by many small hydropower plants may be comparable to or larger than one large hydropower plant.