I have a hard time with this finding the concept for the below: plz help

Question

How and in what way does the input of organic matter, such as sewage, effect the O2 content of a river or stream? A pond or lake?

Does the organic matter get digested by bacteria, microbes et al? Do they multiply rapidly? Do they use oxygen?

As an alternative thought, how does aerobic digestion work in a sewage plant?

As a side point, another outcome of the digestion process is large amounts of carbon dioxide produced, in rivers and ponds, this then results in massive algae formations, which causes environmental problems of its own.