Hydroelectric power falls under the category of a group of resources called alternative or renewable resources.
These include:
                    1. Hydroelectric power- energy stored as potential energy behind a dam
                    2. Geothermal Energy-  energy from the interior of the Earth (the core:geysers, hot springs,or lava)
                    3. Biomass-fermenting of plant matter to create fuels
                    4. Solar Energy- capturing of sunlight to ultimately create electricity
                    5. Wind Energy- the kinetic energy of the wind, having a primary source of the sun ( due to different temperatures in areas)
                    6.  Tidal-the use of  the rising and receding tide
The use of renewable resources is a great alternative to the use of  non-renewable ones. But this does not mean that renewable resources are perfect, as one can see from the following:



Rea source
 Pros
 Cons
Hydro
 Clean
Renewable
Not so noisy
 Can change ecosystem completely, sediment blockage
thus changing the marine environment
Shore line erosion
Short life time
*also see below*
Geothermal
 Renewable
 Extreme temperatures cause materials to melt
( extracting this energy can be difficult)
The life time of such plants can be short
Biomass
 Renewable
 There is a possibility of  excess CO2 production
 (some may dispute this)
Wind
 Clean, Renewable
 Can kill birds(especially birds of prey), Noise
Tidal
 Renewable

 Salt water erodes the machinery
Can change ecosystem


Dams of Hydroelectric Power Plants...
* can change the normal atmospheric pressure in an aquatic environment
          one study found that the pressure in a damed up environment can change from 1atmosphere to 3 or higher.
           Fish living in such a scenario usually cannot adapt to the rapid change in pressure and die.
*increase the amount of gas saturated in the water.
             This is caused by the change in spill. If the water is over saturated with nitrogen it can cause fish to develop gas bubble trauma

Link to article
http://hydropower.inel.gov/turbines/pdfs/becker-repr.pdf


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