Challenges to Further Hydroelectric Deployment

While hydro development is still growing in several regions of the world, many countries, including the United States, will not likely pursue larger hydro projects due to two main factors — first, we have already built hydroelectric dams in most of the best places, and secondly, there are concerns over the environmental and societal impacts of building more dams. These environmental impacts have even been used to justify dam removal in some cases, though weighing those environmental impacts against the societal benefits (such as irrigation, flood control, recreation, and so forth…not just electricity) has always been controversial. Some of these specific environmental and societal impacts include:

Impacts on fish migration – fish migrating up or downstream may find passages blocked by dams or may get killed going through turbines.
Reduced water quality – changes in natural stream-flow may, in some cases, muddy waters. This affects the river ecosystems that aquatic plants and animals utilize.
Land inundation – reservoirs behind dams can flood large areas. There is not necessarily a relationship between the size of the dam and the amount of area flooded behind the dam. Two impacts can arise here. First, building large hydro projects often involves the displacement of many people who used to live in the area surrounding the river. The Three Gorges Dam in China involved the relocation of more than one million people.
Methane release to the atmosphere — plant life can decay in flooded areas, which over time releases large quantities of methane into the atmosphere – enough that it can potentially offset the avoided CO₂ emissions from many years worth of fossil fuel use. A recent estimate places the global emissions of methane high enough to make hydropower be just a little bit less than natural gas in terms of CO2 (equivalent) per unit of energy generated.