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How does human intervention affect the amount of dissolved oxygen that is present in water?

Humans create many conditions that alter the amount of DO in a body of water.  Some of them have little effect, while others have produced disastrous effects on water quality or fish populations.  Human intervention comes in the form of dams, human waste, fertilizer and agricultural waste.  

Dams slow the flow of water, reducing the amount of aeration and increasing the temperature.  In many places these changes will not create bad effects.  However, in other places fish populations are hurt because the dissolved oxygen levels drop below what a fish needs.

Overhead picture of a dam

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