what are 2 advantages and 2 dis-advantages of hydroelectric energy

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Advantages of Hydro-electric energy:  No greenhouse gases, renewable. 

One of the biggest problems we have today is that a large portion of our energy needs come from combustion.  Unfortunately, combustion of anything that contains carbon (about 95% of all combustible material) increases the concentration of carbon dioxide in the atmosphere.  Increasing concentrations of carbon dioxide act like greenhouse windows changing the climate of the Earth.  Using flowing water to turn electric generators doesn't generate carbon dioxide, so it doesn't contribute to climate change. 

Another major problem is that most of our energy sources are non-renewable.  It is believed that transforming organic matter into coal and oil takes thousands of years, yet we use tons of both in just a matter of hours.  Eventually this becomes insustainable, as the coal and oil reserves run out.  Hydropower doesn't have this problem, as the water cycle is very short.  Rain and snow fall in the mountains every year and this water ultimately ends up in the rivers that turn the turbines in the hydro-electric dams.  This makes Hydro-electric energy renewable, like wind and solar power, the water will never entirely run out. 

Disadvantages to hydro-electric energy: it kills fish, it requires swift rivers. 

One of the most contentious debates in the Pacific Northwest is over the hydroelectric dams along the Columbia and Snake rivers.  Since the dams were built in the 1950s, numbers of salmon returning to their Northwest spawning grounds has plummeted.  Research has established that the presence of the dams is at least a partial cause.  Anglers and environmentalist groups want some of these dams removed, to improve the salmon runs.  The dams also contribute to the death of other fish, e.g. trout.  Trout require large amounts of oxygen in the water to live.  Dams destroy many of the "white-water" stretches that oxygenate the water, and contribute to warmer water temperatures which reduces the oxygen's solubility in the water.  This leads to trout being replaced by bass and catfish that can tolerate lower oxygen levels in the water. 

During the Great Mississippi flood of 1992, some people asked, "Where are the flood control dams?"  The answer, there aren't any dams on the Mississippi and Missouri rivers.  Why are there no dams?  The most striking feature of the rivers of the Pacific Northwest is that they have the steepest drops of all rivers in the world.  In fact the Columbia is the steepest river in the world.  It's altitude change is greater than any other river proportional to its length.  This steepness makes the waters flow very fast, and that is crucial to the water having enough energy to turn generators capable of generating a decent amount of electricity.  The rivers of the midwest, and much of the Eastern US don't have this steep altitude drop, and so don't have hydro-electric dams.  The rivers of the West do have the steep drop, and are used for electricity.  Also, the terrain plays a factor.  Most of the rivers in the west flow through deep valleys and canyons, these can be filled with reservoirs behind the dams.  Most of rivers in the midwest meander across vast flat plains.  If you build a dam, the water will just flow around it, which defeats the whole purpose. 

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