How can disolved oxygen effect the water quality of a healthy river?

Question

How can disolved oxygen effect the water quality of a healthy river?

Answer 1

The fish in the river and the smaller fish, insects, crayfish, clams and other items they feed on need to have oxygen for respiration.  If there's no oxygen, there's no life.  Even plants need oxygen because they respire when it dark.  

As the dissolved oxygen level decreases, the sensitive organisms will die or swim (if they're able) to a better location.  So as the oxygen decreases, there will be fewer and fewer types of organisms that can live there.