Thread: What the Wier!

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The Downside of Dams: Is the Environmental Price of Hydroelectric Power Too High? | Scientific American

According to the Hydropower Reform Coalition (HRC), a consortium of 150 groups concerned about the impact of dams, degraded water quality is one of the chief concerns. Organic materials from within and outside the river that would normally wash downstream get built up behind dams and start to consume a large amount of oxygen as they decompose. In some cases this triggers algae blooms which, in turn, create oxygen-starved “dead zones” incapable of supporting river life of any kind. Also, water temperatures in dam reservoirs can differ greatly between the surface and depths, further complicating survival for marine life evolved to handle natural temperature cycling. And when dam operators release oxygen-deprived water with unnatural temperatures into the river below, they harm downstream environments as well.


Temperature
Water held in reservoirs tends to heat up, increasing the temperature of the river. Salmon and steelhead prefer cool water. However, water on the bottom of the reservoir remains cold through the summer. If cool water is taken from the bottom (some dams cannot do this) it may actually be used to help keep the river cool through the summer.

Total dissolved gas
Falling water, such as from a dam spillway, may mix nitrogen from the atmosphere into the water. Water can only hold a certain amount of nitrogen, when this is exceeded, bubbles form. These bubbles can also form inside the bodies of fish which are swimming in the water causing injury or death.

How Dams Affect Water and Habitat on the West Coast | NOAA Fisheries

Oxygen
Water released from the bottom of the reservoir is often low in oxygen, causing problems for fish downstream. However, water falling over a spillway may actually mix more oxygen into the water.

Toxic substances
Sediment settles out in reservoirs behind dams. Many toxic substances may be trapped in these sediments such as pesticides, or heavy metals from mine tailings. If these sediments are disturbed these substances may be released into the water.