Most people have an idea of what good and bad water look like, but we need a specific and consistent definition to use in scientific studies and environmental law. If water supports a beneficial use, water quality is said to be good or unimpaired. If water does not support a beneficial use, water quality is said to be poor or impaired. A key concept is that different beneficial uses have different needs.
Most people believe good water quality means the water is pure and clean.
This is partly true, especially when you are using water for drinking. However, fish and wildlife have lots of other requirements. Fish must get all of their oxygen and food from water, and therefore need water that has enough oxygen and nutrients. All rivers, streams, estuaries, and lakes are assigned to a class based on the beneficial uses they could support if they had good water quality. Water Supply - Domestic, Industrial, Agricultural, and Stock Watering 2.