Question

Where is oxygen content in a river or stream likely to be highest?

Answer 1

where a lot of vegetation is current 

Answer 2

Dissolved oxygen is not as present at the source of a river or stream. As both those bodies of water are free-flowing and moving, that means that such bodies are still receiving ample amounts of water (for example, from a spring or glacier). The source of where the water is coming from is where oxygen content is likely the highest.